DEGRADATION OF MUTANT INFLUENZA-VIRUS HEMAGGLUTININS IS INFLUENCED BYCYTOPLASMIC SEQUENCES INDEPENDENT OF INTERNALIZATION SIGNALS

A mutant influenza virus hemagglutinin, HA+8, having a carboxyl-termin al extension of 8 amino acids that included 4 aromatic residues, was i nternalized within 2 min of arriving at the cell surface and was degra ded quickly by a process that was inhibited by ammonium chloride, Thro ugh second site mutagenesis, the internalization sequence of HA+8 was found to closely resemble the internalization signals of the transferr in receptor or large mannose 6-phosphate receptor, Comparison of the i ntracellular traffic of HA+8 and a series of other HA mutants that dif fered in their rates of internalization revealed a relation between th e amount of the protein on the plasma membrane at steady state and the internalization rate that would be predicted if most of each protein recycled to the cell surface. However, there was no simple correlation between the internalization rate and the rate of degradation, indicat ing that transport to the compartment where degradation occurred was n ot simply a function of the concentration of the proteins in early end osomes, The internal populations of both HA+8, which was degraded with a t(1/2) of 1.9 h, and HA-Y543, which was degraded with a t(1/2) of 2 .9 h, were found by cell fractionation and density-shift experiments t o reside in early endosomes with little accumulation in lysosomes, A f luid-phase marker reached lysosomes 3-4-fold faster than these protein s were degraded, Degradation of these mutant HAs involved a rate-deter mining step in early endosomes that was sensitive to some feature of t he protein that depended upon sequence differences in the cytoplasmic domain unrelated to the internalization signal.