Role of the carboxyl terminus in stable expression of hamster UDP-GlcNAc:dolichol-P GlcNAc-1-P transferase

In order to examine the function of the carboxyl terminus of UDP-GlcNAc:dolichol-P GlcNAc-1-P transferase (GPT), an endoplasmic reticulum enzyme that synthesizes GlcNAc-P-P-dolichol and, thus, catalyzes the committed step for N-linked glycosylation, a series of carboxyl-terminal truncation mutations was examined. Removal of the last 11 amino acids (398-408) from GPT had no significant effect on catalytic activity, thermal stability, tunicamycin binding, reticular localization, or consumption of cellular dolichol-P. However, in the absence of residues 398-408, the removal of three additional residues (Phe³⁹⁵-Ser³⁹⁶-Ile³⁹⁷), or their change to Leu³⁹⁵-Met³⁹⁶-Trp³⁹⁷ fully eliminated enzyme expression in vivo. By reattaching residues 398-408 to Leu³⁹⁵-Met³⁹⁶-Trp³⁹⁷, expression was restored. Thus, the carboxyl-terminal region of GPT is essential for stable expression. Either of two sequences (395-397 and 398-408) is sufficient for expression, but neither is necessary. Expression of GPT in the absence of residues 398-408 specifically required the Phe³⁹⁵-Ser³⁹⁶-Ile³⁹⁷ sequence, since most scramble and termination mutations within this sequence were inhibitory. One scramble mutant (Ile³⁹⁵-Ser³⁹⁶-Phe³⁹⁷-Stop³⁹⁸) was enzymatically active, but unusually thermolabile. Thus, the function of Phe³⁹⁵-Ser³⁹⁶Ile³⁹⁷ may be to stabilize GPT.