Calcium ion-dependent vesicle fusion in the conversion of proalbumin to albumin

Abstract

PROALBUMIN is the immediate biosynthetic precursor of albumin¹, and has been shown (in the rat) to contain a N-terminal extension (Arg-Gly-Val-Phe-Arg-Arg)^2,3 which can be cleaved by trypsin to yield albumin. We have recently found an enzyme in rat liver, tentatively identified as cathepsin B (ref. 4), which converts proalbumin to albumin, and which is inhibited 70% by leupeptin (3 µM) and by tosyl-lysyl-chloro-methyl ketone (TLCK, 10 µM). Purified bovine cathepsin B (provided by Dr A. J. Barrett) has also been found to carry out the conversion of proalbumin. Zühlke et al.⁵ have discussed the possibility that cathepsin B might catalyse the conversion of proinsulin; and since double basic residues occur at the cleavage points of a number of proproteins, a common enzymatic mechanism might be involved in their conversion for secretion. Our earlier work⁶ suggested that conversion of proalbumin was a very late event in the secretory pathway, which agrees with the suggestion of Ikehara et al.⁷ that conversion occurs in Golgi vesicles in vivo. Here, we present evidence that conversion takes place in isolated Golgi vesicles in vitro, that the enzyme concerned is probably cathepsin B, and that the conversion of proalbumin to albumin involves a Ca²⁺-requiring intracellular vesicle fusion occurring at or beyond a transit step through the Golgi apparatus.