The Membrane Integration of the Hemagglutinin-Neuraminidase Glycoprotein of Newcastle Disease Virus: A Thesis

Abstract

The hemagglutinin-neuraminidase (HN) molecule of Newcastle disease virus (NDV) is an integral membrane glycoprotein that is oriented with its N-terminus in the cytoplasm and its C-terminus external to the infected cell. Single spanning membrane proteins with this type of topology (N-terminus in, C-terminus out) have been classified as Type II glycoproteins, in contrast to the more common Type I glycoproteins, which are oriented in the opposite direction. (C-terminus in, N-terminus out). The membrane integration of HN protein was investigated using a wheat germ translation system to synthesize and integrate HN protein into microsomal membranes in vitro. The insertion and translocation of HN protein into microsomal vesicles was found to occur cotranslationally without signal sequence cleavage. The membrane targeting required both signal recognition particle (SRP) and SRP receptor. Membrane binding assays utilizing HN nascent chain/ribosome/SRP complexes demonstrated that the membrane insertion of HN polypeptide required the presence of GTP, in a way similar to that described for secretory, multispanning and Type I proteins. To investigate further the membrane translocation process of HN protein, the amino terminal region of HN was mutated to determine the role of this region in the membrane integration of HN. The cDNA sequence encoding the bulk of the cytoplasmic tail of the HN glycoprotein was deleted. When transcripts produced from the mutated cDNA were translated in wheat germ extract in the presence of membranes, several abnormalities were identified in the interaction of the mutant protein with membranes. Although translocation and glycosylation of the mutant protein was detected, the efficiency of membrane translocation and the stability of the mutant protein's membrane interaction were reduced. Even though a large proportion of the mutant products remained nontranslocated and unglycosylated, many of these products were inserted into membrane vesicles in a reverse orientation from the wild type HN protein. The aberrant insertion of the mutant protein required both SRP and SRP receptor. Ribosome-bound mutant nascent chains were able to insert into membranes without the addition of GTP or SRP, but this GTP-independent insertion was in reverse. Therefore, the cytoplasmic tail of the HN glycoprotein appears to playa critical role in the maintanence of faithful directionality of the protein's membrane insertion.