IDENTIFICATION OF A SEGMENT OF THE SMALL NUCLEOLAR RIBONUCLEOPROTEIN-ASSOCIATED PROTEIN GAR1 THAT IS SUFFICIENT FOR NUCLEOLAR ACCUMULATION

GAR1 is a 25-kDa nucleolar protein that is essential for yeast cell gr owth. The protein is associated with a subset of small nucleolar RNAs and is required for pre-rRNA processing. By expressing in yeast variou s deletions of GAR1 fused to a reporter protein, we have searched for which particular domain of GAR1 can account for its nucleolar localiza tion. We report here that the glycine/arginine-rich domains of GAR1, w hich are shared by several other nucleolar proteins, are neither suffi cient nor required for the steady-state accumulation of the fusion pro tein in the nucleolus. We further demonstrate that the central domain of GAR1 is both sufficient to target the beta-galactosidase to the yea st nucleolus and to restore the growth of a strain deficient in GAR1. As opposed to the other characterized nucleolar proteins, the nucleola r targeting domain of GAR1 does not exhibit any homology with the SV40 T-antigen-type nuclear localization sequence. Moreover, none of the m odified GAR1 proteins that we examined has allowed us to distinguish t he nuclear and nucleolar targeting domains. The presence in GAR1 of a single domain that is responsible for both nuclear entry and nucleolar accumulation suggests that GAR1 either could be carried piggyback by another nucleolar component, possibly as part of a small nucleolar rib onucleoprotein particle, or could be transported to the nucleolus by u sing a pathway different from the other nucleolar proteins.