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L M Hendershot

Publications and source records attributed to L M Hendershot.

6 recordsLinked to original sources
PubMed1992

Interconversion of three differentially modified and assembled forms of BiP.

The immunoglobulin heavy chain binding protein BiP/GRP78 is post-translationally modified by phosphorylation and ADP ribosylation. In cells induced to synthesize higher levels of BiP, either due to the accumulation of nontransported proteins or to glucose starvation, both BiP phosphorylation and ADP ribosylation are reduced. BiP bound to other proteins is unmodified, suggesting that both phosphorylation and ADP ribosylation are restricted to the unbound BiP pool. In the present study, both modifications were further characterized in terms of their stability, the pool of BiP that harbored these modifications, and the relationship between the modified and unmodified forms of BiP. While levels of BiP synthesis vary according to the physiological state of a cell, we found that both induced and uninduced cells contain similar amounts of free BiP. However, free BiP in uninduced cells was found primarily in an aggregated state, whereas in cells that accumulate nontransported proteins, it was predominantly monomeric. Both phosphorylation and ADP ribosylation were restricted to the aggregated form of free BiP. These post-translational modifications occurred upon release of BiP from associated proteins, and could be reversed upon induction of BiP synthesis. Therefore, BiP exists either (1) complexed to other proteins, (2) as a free unmodified monomer, or (3) as free modified aggregates. Our data suggest that BiP can be interconverted from one state to another, and that the various forms are functionally distinct.

Adenosine Diphosphate↗
PubMed1991

Regulation of IgM and IgD expression in human B-lineage cells.

IgD is thought to function primarily as an Ag receptor that is expressed, together with IgM, only on mature B lymphocytes. This differentiation stage-specific expression of IgD has been well characterized in mice, where delta mRNA is detected only in mature IgM/IgD B cells. Humans, in contrast to mice, have significant levels of serum IgD, suggesting that the regulation of this isotype might differ between the two species. Therefore, we examined the regulation of both IgM and IgD expression in cell lines encompassing the spectrum of human B lineage development. Surprisingly, two species of delta mRNA could be found at all differentiation stages -from mu+ pre-B cell to IgM-secreting plasmablast. These mRNA are translated to yield the membrane and secretory forms of delta. The membrane delta-chain: secretory delta-chain ratio did not necessarily reflect the membrane mu-chain:secretory mu-chain ratio in the same cell line, implying that different mechanisms are involved in the selection of membrane vs secretory mu- and delta-chains. The delta-chains synthesized in pre-B cells were degraded, but in more mature cell types IgD could be stably expressed and secreted. Exceptions to this panlineage synthesis of delta-chains were, however, observed in two of the B cell lymphomas, where delta expression was prevented by transcriptional and posttranscriptional mechanisms. The presence of delta-chain in pre-B cells and the secretion of IgD by more mature cells suggest that IgD may have immunoregulatory roles throughout B cell differentiation. These studies also indicated that the bias toward secretory mu-chain production that occurs in human IgM secreting cells results from posttranscriptional regulation. In addition, we have identified a B cell line that synthesizes both normal-sized mu-chains and those with smaller apparent m.w. translation products of truncated mu mRNA.

B-Lymphocytes↗
PubMed1990

Association of transport-defective light chains with immunoglobulin heavy chain binding protein.

Immunoglobulin light chains are usually secreted from cells when they are synthesized alone or in molar excess of heavy chains, but, there have been reports of nonsecreted light chains. We wished to determine whether immunoglobulin heavy chain binding protein (BiP), which blocks the transport of free heavy chains, might be responsible for the lack of secretion of some light chains. In two murine lymphoid cell lines that synthesize but do not secrete immunoglobulin light chains, the free light chain polymers were found bound to BiP. Examination of 20 other cell lines and hybridomas failed to disclose any cells synthesizing free or excess light chains that associated with BiP, in all cases the free light chains were secreted as dimers. Despite their association with BiP and their blocked secretion, the aberrant light chains could combine with heavy chains and could be secreted as intact Ig molecules. Thus, while light chains do not usually express signals which allow them to bind to BiP, it appears that such signals can be expressed on certain light chains, resulting in their combination with BiP and blocked secretion. When single chain mutant cell lines are isolated from parental lines producing both heavy and light chains, they are almost always light chain producers suggesting that free heavy chains are much more toxic than free light chains. In both PC700 and P3X63Ag cells, however, clones that have lost either heavy chains or transport-defective light chains are present at the same frequency. Our findings that the light chains in both of these lines are associated with BiP raise the possibility that BiP actually contributes to heavy chain toxicity instead of preventing it.

Animals↗
PubMed1990

Immunoglobulin heavy chain and binding protein complexes are dissociated in vivo by light chain addition.

Immunoglobulin heavy chain binding protein (BiP, GRP78) associates stably with the free, nonsecreted Ig heavy chains synthesized by Abelson virus transformed pre-B cell lines. In cells synthesizing both Ig heavy and light chains, the Ig subunits assemble rapidly and are secreted. Only incompletely assembled Ig molecules can be found bound to BiP in these cells. In addition to Ig heavy chains, a number of mutant and incompletely glycosylated transport-defective proteins are stably complexed with BiP. When normal proteins are examined for combination with BiP, only a small fraction of the intracellular pool of nascent, unfolded, or unassembled proteins can be found associated. It has been difficult to determine whether these BiP-associated molecules represent assembly intermediates which will be displaced from BiP and transported from the cell, or whether these are aberrant proteins that are ultimately degraded. In order for BiP to monitor and aid in normal protein transport, its association with these proteins must be reversible and the released proteins should be transport competent. In the studies described here, transient heterokaryons were formed between a myeloma line producing BiP-associated heavy chains and a myeloma line synthesizing the complementary light chain. Introduction of light chain synthesis resulted in assembly of prelabeled heavy chains with light chains, displacement of BiP from heavy chains, and secretion of Ig into the culture supernatant. These data demonstrate that BiP association can be reversible, with concordant release of transportable proteins. Thus, BiP can be considered a component of the exocytic secretory pathway, regulating the transport of both normal and abnormal proteins.

Animals↗
PubMed1989

Mu heavy chains can associate with a pseudo-light chain complex (psi L) in human pre-B cell lines.

In pre-B cells, the earliest identifiable stage of B cell differentiation, there is an asynchrony of immunoglobulin chain expression in that mu heavy chains are synthesized in the absence of light chain synthesis. These mu chains largely remain intracellular and are degraded. Here we demonstrate that a fraction of mu chains in human pre-B cell lines can reach the surface in association with three pre-B-specific proteins with relative molecular masses of 22, 18, and 16 kd, which we term collectively the pseudo-light chain complex, psi L. This association generates a multimeric complex, mu 2-psi L. Two of the psi L proteins (22 and 16 kd) are lambda-immunoreactive and form disulfide bonds with mu chains, suggesting that they are closely related to conventional lambda light chains. The 18 kd psi L species is a non-covalently-associated member of the complex. The expression of mu-psi L complexes on the surface of pre-B cells could have a functional role in the control of pre-B growth and differentiation by the hematopoietic microenvironment.

B-Lymphocytes↗