Fusion proteins with COOH-terminal ubiquitin are stable and maintain dual functionality in vivo

The ubiquitin (Ub) fusion degradation pathway functions to degrade fusion proteins containing a nonremovable Ub moiety at their NH(2) terminus (Johnson, E. S., Ma, P. C., Ota, I. M., and Varshavsky, A. (1995) J. Biol. Chem. 270, 17442-17456). Here we show that ubiquitin fusion degradation also targets proteins for proteasomal degradation when Ub is present in the middle of fusion proteins (X-Ub-Y), in a process that entails polyubiquitylation of Ub Lys(48). By contrast, fusion proteins bearing COOH-terminal Ub (X-Ub) are metabolically stable. Such fusion proteins, either newly biosynthesized or generated by Ub hydrolases, are reversibly conjugated to heterogeneous target proteins in a manner similar to wild-type Ub. Most importantly, the NH(2)-terminal fusion partner (X) can maintain its structure and function in the formed X-Ub conjugates as inferred from the fluorescence of green fluorescent protein-Ub conjugates and the incorporation of human immunodeficiency virus type 1 Gag-Ub into viral particles. These findings strongly suggest that 26S proteasomes exhibit spatial discrimination of Ub-conjugated proteins, sparing domains extended from the NH(2) terminus of Ub from unfolding and degradation. The multifunctionality of X-Ub fusion proteins opens the possibility for a number of novel practical applications, including the imaging of Ub conjugate formation in living cells.