BIOSYNTHESIS, DISTINCT POSTTRANSLATIONAL MODIFICATIONS, AND FUNCTIONAL-CHARACTERIZATION OF LYMPHOMA PROPROTEIN CONVERTASE

Proprotein convertases are responsible for the endoproteolytic process ing of prohormones, neuropeptide precursors, and other proproteins wit hin the constitutive and regulated secretory pathways. Cleavage occurs carboxyl-terminally of basic amino acid motifs, such as RX(K/R)R, RXX R, and (R/K)R. As already available for the other known mammalian memb ers of this enzyme family, we here define structural and functional fe atures of human lymphoma proprotein convertase (LPC). Analysis of expr ession of recombinant LPC in stably transfected Chinese hamster ovary cells reveals biosynthesis of a 92-kDa nonglycosylated precursor (proL PC) and a 102-kDa endoglycosidase H-sensitive glycosylated form of pro LPC. Only the latter is further processed and after propeptide removal converted into a complexly N-glycosylated mature form of LPC of about 92 kDa. Co-expression experiments of truncated LPC with an active sit e mutant of LPC (LPCS265A) indicate that prodomain removal of LPC occu rs via an autoproteolytic, intramolecular mechanism, as was demonstrat ed before for some of the other members of this enzyme family. Prodoma in removal is shown to be required for LPC to exit the endoplasmic ret iculum. As far as subcellular localization is concerned, immunocytoche mical, ultrastructural, and biochemical analyses show that LPC is conc entrated in the trans-Golgi network, associated with membranes, and no t secreted. Carboxyl-terminal domains are critically involved in this cellular retention, because removal of both the hydrophobic region and the cytoplasmic tail of LPC results in secretion. Of interest are the observations that LPC is not phosphorylated like furin but is palmito ylated in its cytoplasmic tail. Finally, substrate specificity of LPC is similar to that of furin but not identical. Whereas for furin a bas ic substrate residue at position P-2 is dispensable, it is essential f or LPC. For optimal LPC substrate processing activity, an arginine at position P-6 is preferred over an arginine at P-4.