Biomedical subjects

Suraj Saksena

Publications and source records attributed to Suraj Saksena.

5 recordsLinked to original sources

PubMed2006

New component of ESCRT-I regulates endosomal sorting complex assembly.

The endosomal sorting complex required for transport (ESCRT) complexes play a critical role in receptor down-regulation and retroviral budding. Although the crystal structures of two ESCRT complexes have been determined, the molecular mechanisms underlying the assembly and regulation of the ESCRT machinery are still poorly understood. We identify a new component of the ESCRT-I complex, multivesicular body sorting factor of 12 kD (Mvb12), and demonstrate that Mvb12 binds to the coiled-coil domain of the ESCRT-I subunit vacuolar protein sorting 23 (Vps23). We show that ESCRT-I adopts an oligomeric state in the cytosol, the formation of which requires the coiled-coil domain of Vps23, as well as Mvb12. Loss of Mvb12 results in the disassembly of the ESCRT-I oligomer and the formation of a stable complex of ESCRT-I and -II in the cytosol. We propose that Mvb12 stabilizes ESCRT-I in an oligomeric, inactive state in the cytosol to ensure that the ordered recruitment and assembly of ESCRT-I and -II is spatially and temporally restricted to the surface of the endosome after activation of the MVB sorting reaction.

Cell Compartmentation↗

PubMed2006

Importin-alpha-16 is a translocon-associated protein involved in sorting membrane proteins to the nuclear envelope.

A viral inner nuclear membrane-sorting motif sequence (INM-SM) was used to identify proteins that recognize integral membrane proteins destined for the INM. Herein we describe importin-alpha-16, a membrane-associated isoform of Spodoptera frugiperda importin-alpha that contains the C-terminal amino acid residues comprising armadillo helical-repeat domains 7-10. In the endoplasmic reticulum (ER) membrane, importin-alpha-16 is adjacent to the translocon protein Sec61alpha. Importin-alpha-16 cross-links to the INM-SM sequence as it emerges from the ribosomal tunnel and remains adjacent to the INM-SM after INM-SM integration into the ER membrane and release from the translocon. Cross-linking results suggest that importin-alpha-16 discriminates between INM- and non-INM-directed proteins. Thus, it seems that during and after cotranslational membrane integration, importin-alpha-16 is involved in the trafficking of integral membrane proteins to the INM.

Amino Acid Sequence↗

PubMed2005

Double-spanning plant viral movement protein integration into the endoplasmic reticulum membrane is signal recognition particle-dependent, translocon-mediated, and concerted.

The current model for cell-to-cell movement of plant viruses holds that transport requires virus-encoded movement proteins that intimately associate with endoplasmic reticulum membranes. We have examined the early stages of the integration into endoplasmic reticulum membranes of a double-spanning viral movement protein using photocross-linking. We have discovered that this process is cotranslational and proceeds in a signal recognition particle-dependent manner. In addition, nascent chain photocross-linking to Sec61alpha and translocating chain-associated membrane protein reveal that viral membrane protein insertion takes place via the translocon, as with most eukaryotic membrane proteins, but that the two transmembrane segments of the viral protein leave the translocon and enter the lipid bilayer together.

Calcium-Binding Proteins↗

PubMed2004

Cotranslational integration and initial sorting at the endoplasmic reticulum translocon of proteins destined for the inner nuclear membrane.

The current diffusion-retention model for protein trafficking to the inner nuclear membrane (INM) proposes that INM proteins diffuse laterally from the membrane of the endoplasmic reticulum into the INM and are then retained in the INM by binding to nuclear proteins or DNA. Because some data indicate that the sorting of baculovirus envelope proteins to the INM is protein-mediated, we have examined the early stages of INM protein integration and sorting by using photocrosslinking. Both viral and host INM-directed proteins were integrated cotranslationally through the endoplasmic reticulum translocon, and their nonrandom photocrosslinking to two translocon proteins, Sec61alpha and translocating chain-associated membrane protein (TRAM), revealed that the first transmembrane sequence (TMS) of each viral and host INM-directed protein occupied a very similar location within the translocon. Because few TMSs of non-INM-directed membrane proteins photocrosslink to TRAM, it seems that the INM-directed TMSs occupy different sites within the translocon than do non-INM-directed TMSs. The distinct proximities of translocon components to INM-directed TMSs strongly suggest that such TMSs are recognized and initially sorted within the translocon. Taken together, these data indicate that membrane protein sorting to the INM is an active process involving specific nonnuclear proteins.

Amino Acid Sequence↗

PubMed2004

Trafficking of ODV-E66 is mediated via a sorting motif and other viral proteins: facilitated trafficking to the inner nuclear membrane.

The N-terminal 33 aa of the envelope protein ODV-E66 are sufficient to traffic fusion proteins to intranuclear membranes and the ODV envelope during infection with Autographa californica nucleopolyhedrovirus. This sequence has two distinct features: (i) an extremely hydrophobic sequence of 18 aa and (ii) positively charged amino acids close to the C-terminal end of the hydrophobic sequence. In the absence of infection, this sequence is sufficient to promote protein accumulation at the inner nuclear membrane. Covalent cross-linking results show that the lysines of the motif are proximal to FP25K and/or BV/ODV-E26 during transit from the endoplasmic reticulum to the nuclear envelope. We propose that the 33 aa comprise a signature for sorting proteins to the inner nuclear membrane (sorting motif) and that, unlike other resident proteins of the inner nuclear membrane, ODV-E66 and sortingmotif fusions do not randomly diffuse from their site of insertion at the endoplasmic reticulum to the nuclear envelope and viral-induced intranuclear membranes. Rather, during infection, trafficking is mediated by protein-protein interactions.

Amino Acid Sequence↗