A stress-regulated protein, GRP58, a member of thioredoxin superfamily, is a carnitine palmitoyltransferase isoenzyme.

We recently noted the association of carnitine palmitoyltransferase (CPT) activity with a 54 kDa microsomal protein [Murthy and Pande (1993) Mol. Cell Biochem. 122, 133-138] that, based on amino-acid-sequence identity, seemed to be the protein previously described as a 'glucose-regulated protein-58' (GRP58), phosphoinositide-specific phospholipase C, hormone-induced protein-70, endoplasmic-reticulum protein-61 (ERp61), protein disulphide-isomerase, thiol protease, a protein affected in halothane anaesthesia and one that affects renal-tubular functions and the transcriptional activation of the interferon-alpha inducible genes. To ascertain the catalytic identity of this protein unambiguously, we have expressed the corresponding cDNA transiently and stably in human kidney 293 cells as well as in HeLa cells. In each case we found that expression led to an increase in assayable and immunoreactive 54 kDa CPT activity, whereas the protein disulphide-isomerase activity was not increased. In vitro expression in a cell-free transcription and translation system led to the synthesis of a approximately 57 kDa (precursor) protein that was processed to a approximately 54 kDa (mature) protein when microsomes were present; in both these experiments again a large increase in CPT activity was seen. Thus the present data provide compelling evidence that the 54 kDa protein in question is a CPT isoenzyme. It remains to be seen now how the ability of this protein to interconvert acyl-CoA and acylcarnitine would relate to the diverse functions indicated for this protein in vivo.