Biomedical subjects

K Mahlke

Publications and source records attributed to K Mahlke.

4 recordsLinked to original sources

PubMed1992

The cleavable presequence is not essential for import and assembly of the phosphate carrier of mammalian mitochondria but enhances the specificity and efficiency of import.

The phosphate carrier (PiC) of mammalian mitochondria is synthesized with a cleavable presequence, in contrast to other members of the mitochondrial family of inner membrane carrier proteins. The precursor of PiC is efficiently imported, proteolytically processed, and correctly assembled in isolated mitochondria. Here we report that a presequence-deficient PiC was imported with an efficiency of about 50% as compared with the authentic precursor of PiC. This mature-sized PiC was correctly assembled, demonstrating that the presequence is not essential for the assembly pathway. We found the following functions for the PiC presequence. (i) The presequence by itself was able to target a passenger protein to mitochondria with a low efficiency, suggesting that the mammalian PiC contains multiple targeting signals, the more efficient one(s) present in the mature protein part. (ii) Deletion of the presequence allowed a more efficient heterologous import of mammalian PiC into mitochondria from Saccharomyces cerevisiae and Neurospora crassa, indicating an important role of the presequence in determining the specificity of PiC import. (iii) Import of the presequence-deficient PiC required a higher membrane potential across the inner membrane than that of the presequence-carrying form. Therefore, the presequence also enhances the translocation of PiC into the inner membrane.

Animals↗

PubMed1991

Role of an energized inner membrane in mitochondrial protein import. Delta psi drives the movement of presequences.

The transport of precursor proteins into mitochondria requires an energized inner membrane. We report here that the import of various precursor proteins showed a differential sensitivity to treatment of the mitochondria with the uncoupler carbonyl cyanide m-chlorophenylhydrazone. The differential inhibition by carbonyl cyanide m-chlorophenylhydrazone was not influenced by the length of the precursor, the presence of mature protein parts, or the folding state of the precursor but was specific for the presequence. Moreover, only the membrane potential delta psi and not the total proton motive force was required for the transport of precursors, indicating that protein translocation across the inner membrane is not driven by a movement of protons. We conclude that delta psi (negative inside) is needed for the translocation of the positively charged presequences, possibly via an electrophoretic effect.

Biological Transport↗

PubMed1990

Sorting pathways of mitochondrial inner membrane proteins.

Two distinct pathways of sorting and assembly of nuclear-encoded mitochondrial inner membrane proteins are described. In the first pathway, precursor proteins that carry amino-terminal targeting signals are initially translocated via contact sites between both mitochondrial membranes into the mitochondrial matrix. They become proteolytically processed, interact with the 60-kDa heat-shock protein hsp60 in the matrix and are retranslocated to the inner membrane. The sorting of subunit 9 of Neurospora crassa F0-ATPase has been studied as an example. F0 subunit 9 belongs to that class of nuclear-encoded mitochondrial proteins which are evolutionarily derived from a prokaryotic ancestor according to the endosymbiont hypothesis. We suggest that after import into mitochondria, these proteins follow the ancestral sorting and assembly pathways established in prokaryotes (conservative sorting). On the other hand, ADP/ATP carrier was found not to require interaction with hsp60 for import and assembly. This agrees with previous findings that the ADP/ATP carrier possesses non-amino-terminal targeting signals and uses a different import receptor to other mitochondrial precursor proteins. It is proposed that the ADP/ATP carrier represents a class of mitochondrial inner membrane proteins which do not have a prokaryotic equivalent and thus appear to follow a non-conservative sorting pathway.

Base Sequence↗

PubMed1984

Androgen concentrations in testicular and peripheral blood in the male Japanese quail.

Two experiments were conducted in which the concentrations of testosterone and dihydrotestosterone (DHT), an active metabolite of testosterone, were studied in the general circulation and in the testicular effluent. In the first experiment, blood samples taken from adult male quail were analyzed by radioimmunoassay for androgen and DHT concentrations; from these data, testosterone concentrations were calculated. The average concentration of androgen was 4238 pg/ml plasma. The average ratio of DHT/testosterone was .75. In the second experiment, steroid concentrations were compared in blood samples taken from the testicular vein and from the general circulation. Androgen concentrations in the general circulation were 36% of that obtained for the testicular effluent. In addition, testosterone and DHT concentrations in the general circulation were 36 and 46%, respectively, as high as concentrations in the testicular vein. Comparison of the average ratios of testosterone/DHT in the general versus testicular plasma revealed a higher ratio in the general circulation (.73 vs. .51, respectively). These results provide additional evidence for testosterone as the major male steroid secreted by the gonads. In addition, the relative concentration of DHT in the quail, which is higher than documented for most species, was higher in the peripheral than in the testicular circulation. This may indicate peripheral metabolism of testosterone to its active metabolite, DHT, and could have implications as to the availability of DHT for its target tissues.

Androgens↗