P. Slusarewicz et al., I-2(B) IS A SMALL CYTOSOLIC PROTEIN THAT PARTICIPATES IN VACUOLE FUSION, Proceedings of the National Academy of Sciences of the United Statesof America, 94(11), 1997, pp. 5582-5587
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).