I-2(B) IS A SMALL CYTOSOLIC PROTEIN THAT PARTICIPATES IN VACUOLE FUSION

Saccharomyces cerevisiae vacuole inheritance requires two low molecula r weight activities, LMA1 and LMA2. LMA1 is a heterodimer of thioredox in and protease B inhibitor 2 (I-2(B)). Here we show that the second l ow molecular weight activity (LMA2) is monomeric I-2(B). Though LMA2/I -2(B) was initially identified as a protease B inhibitor, this proteas e inhibitor activity is not related to its ability to promote vacuole fusion: (i) Low M-r protease B inhibitors cannot substitute for LMA1 o r LMA2, (ii) LMA1 and LMA2 promote the fusion of vacuoles from a strai n that has no protease B, (iii) low concentrations of LMA2 that fully inhibit protease B do not promote vacuole fusion, and (iv) LMA1, in wh ich I-2(B) is complexed with thioredoxin, is far more active than LMA2 /I-2(B) in promoting vacuole fusion and far less active in inhibiting protease B. These studies establish a new function for I-2(B).