The hemagglutinin-neuraminidase (HN) protein of Newcastle disease virus (NDV) is a type II glycoprotein oriented in the plasma membrane with its amino terminus in the cytoplasm and its carboxy terminus external to the cell. We have previously shown that the membrane insertion of HN protein requires signal recognition particle SRP, occurs cotranslationally, and utilizes the same GTP-dependent step that has been described for secretory proteins, type I proteins, and multispanning proteins (C. Wilson, R. Gilmore, and T. Morrison, Mol. Cell. Biol. 7:1386-1392, 1987; C. Wilson, T. Connolly, T. Morrison, and R. Gilmore, J. Cell Biol. 107:69-77, 1988). The role of the amino-terminal cytoplasmic domain in the faithful membrane insertion of this type II protein was explored by characterizing the membrane integration of a mutant lacking 23 of the 26 amino acids of the cytoplasmic domain. The mutant protein was able to interact with SRP, resulting in translation inhibition, membrane targeting, and membrane translocation, but the efficiency of translocation was considerably lower than for the wild-type HN protein. In addition, a significant proportion of the mutant protein synthesized in the presence of SRP and microsomal membranes was associated with the membrane in an EDTA- and alkali-insensitive manner yet integrated into membranes with its carboxy-terminal domain on the cytoplasmic side of membrane vesicles. Membrane-integrated molecules with this reverse orientation were not detected when the mutant protein was synthesized in the absence of SRP or a functional SRP receptor. Truncated mRNAs encoding amino-terminal segments of the wild-type and mutant proteins were translated to prepare ribosomes bearing arrested nascent chains. The arrested mutant nascent chain, in contrast to the wild-type nascent chain, was also able to insert into membranes in a GTP- and SRP-independent manner. Results suggest that the cytoplasmic domain plays a role in the proper membrane insertion of this type II glycoprotein.

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.

We have examined the requirement for ribonucleotides and ribonucleotide triphosphate hydrolysis during early events in the membrane integration of two membrane proteins: the G protein of vesicular stomatitis virus and the hemagglutinin-neuraminidase (HN) glycoprotein of Newcastle disease virus. Both proteins contain a single transmembrane-spanning segment but are integrated in the membrane with opposite orientations. The G protein has an amino-terminal signal sequence and a stop-transfer sequence located near the carboxy terminus. The HN glycoprotein has a single sequence near the amino terminus that functions as both a signal-sequence and a transmembrane-spanning segment. Membrane insertion was explored using a cell-free system directed by transcribed mRNAs encoding amino-terminal segments of the two proteins. Ribosome-bound nascent polypeptides were assembled, ribonucleotides were removed by gel filtration chromatography, and the ribosomes were incubated with microsomal membranes under conditions of defined ribonucleotide content. Nascent chain insertion into the membrane required the presence of both the signal recognition particle and a functional signal recognition particle receptor. In the absence of ribonucleotides, insertion of nascent membrane proteins was not detected. GTP or nonhydrolyzable GTP analogues promoted efficient insertion, while ATP was comparatively ineffective. Surprisingly, the majority of the HN nascent chain remained ribosome associated after puromycin treatment. Ribosome-associated HN nascent chains remained competent for membrane insertion, while free HN chains were not competent. We conclude that a GTP binding protein performs an essential function during ribosome-dependent insertion of membrane proteins into the endoplasmic reticulum that is unrelated to protein synthesis.

The hemagglutinin-neuraminidase (HN) protein of paramyxoviruses is likely in the unusual class of glycoproteins with the amino terminus cytoplasmic and the carboxy terminus lumenal or external to the cell. The properties of the membrane insertion of the HN protein of Newcastle disease virus, a prototype paramyxovirus, were explored in wheat germ extracts containing microsomal membranes. HN protein was inserted into membranes cotranslationally, resulting in a glycosylated protein completely resistant to trypsin and proteinase K digestion. No detectable posttranslation insertion occurred. Insertion required signal recognition particle. Signal recognition particle in the absence of membranes inhibited HN protein synthesis. Comparisons of the trypsin digestion products of the HN protein made in the cell-free system with newly synthesized HN protein from infected cells showed that the cell-free product was in a conformation different from that of the pulse-labeled protein in infected cells. First, trypsin digestion of intact membranes from infected cells reduced the size of the 74,000-dalton HN protein by approximately 1,000 daltons, whereas trypsin digestion of HN protein made in the cell-free system had no effect on the size of the protein. Second, trypsin digestion of Triton X-100-permeabilized membranes isolated from infected cells resulted in a 67,000-dalton trypsin resistant HN protein fragment. A trypsin-resistant core of comparable size was not present in the digestion products of in-vitro-synthesized HN protein. Evidence is presented that the newly synthesized HN protein in infected cels contain intramolecular disulfide bonds not present in the cell-free product.