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 growth. The protein is associated with a subset of small nucleolar RNAs and is required for pre-rRNA processing. By expressing in yeast various deletions of GAR1 fused to a reporter protein, we have searched for which particular domain of GAR1 can account for its nucleolar localization. We report here that the glycine/arginine-rich domains of GAR1, which are shared by several other nucleolar proteins, are neither sufficient nor required for the steady-state accumulation of the fusion protein in the nucleolus. We further demonstrate that the central domain of GAR1 is both sufficient to target the beta-galactosidase to the yeast nucleolus and to restore the growth of a strain deficient in GAR1. As opposed to the other characterized nucleolar proteins, the nucleolar targeting domain of GAR1 does not exhibit any homology with the SV40 T-antigen-type nuclear localization sequence. Moreover, none of the modified GAR1 proteins that we examined has allowed us to distinguish the 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 ribonucleoprotein particle, or could be transported to the nucleolus by using a pathway different from the other nucleolar proteins.