Regulatory events in the synthesis of Sendai virus polypeptides and their assembly into virions

De novo synthesis of N and P proteins as a key step in Sendai virus gene expression

Among the members of the paramyxovirus family, the transcription process and the components involved have been studied under in vitro conditions thus far. Here, we reexamined the function of the viral RNA-dependent RNA polymerase through infection studies with Sendai virus (SeV) N and P deletion (Delta) mutants. To elucidate solely transcription-specific processes, all virus mutants also were rendered deficient in genome replication. Using mutant SeV DeltaP, the earlier suspected supplemental role of P protein was clearly demonstrated to be essential during viral gene expression. Moreover, when SeV DeltaN or DeltaN PDelta2-77 (with the 5' end of the P gene deleted) mutant was used for infections, a completely unexpected new and essential role for N protein was discovered for viral gene expression. In the early phases of an infection and in the absence of de novo viral protein synthesis, primary transcription occurs at hardly detectable levels. In contrast, if newly synthesized N protein is present, primary viral transcription reaches normal levels. From our data, we conclude that de novo synthesis of SeV N and P proteins is a key step for viral gene expression that facilitates the transition from preliminary to normal primary transcriptional activity.

Nucleocapsid formation and/or subsequent conformational change of rabies virus nucleoprotein (N) is a prerequisite step for acquiring the phosphatase-sensitive epitope of monoclonal antibody 5-2-26

We investigated the antigenic maturation of rabies virus N protein, for which we used some conformational epitope-specific monoclonal antibodies (MAbs) and an MAb (5-2-26) against a phosphorylation-dependent linear epitope. Infected cells were lysed with a deoxycholate-free lysis buffer and separated by ultracentrifugation into the soluble top and the nucleocapsid fractions. None of the study MAbs recognized N proteins in the top fraction, whereas nucleocapsid-associated N proteins were recognized by all of the MAbs. Immunoprecipitation with polyclonal anti-N antibodies coprecipitated the P proteins from the top fraction, indicating that soluble N proteins are mostly associated with the P protein. The N proteins dissociated from both the N-P complex and nucleocapsids were recognized by none of the study MAbs, whereas the MAb 5-2-6 recognized the SDS-denatured N proteins of the nucleocapsid but not of the top fraction. In addition, the phosphorylation-deficient mutant N proteins were shown to be similarly accumulated as the wild-type N proteins into the viral inclusion bodies, defined as the virus-specific structures composed of viral nucleocapsids, that are produced in the cytoplasm of the infected cells. Based on these results, we believe that newly synthesized N proteins are not immediately phosphorylated at serine-389 (a common phosphorylation site) but are first associated with the P protein. After being used for encapsidation of the viral RNA, the N proteins undergo conformational changes, whereby epitopes for the conformation-specific MAbs are formed and become phosphorylated at serine-389.

Conformational maturation of measles virus nucleocapsid protein

We have obtained a polyclonal antiserum, N-BE, against the denatured, amino-terminal half of the measles virus (MV) nucleocapsid (N) protein and a monoclonal antibody (MAb), N46, which recognizes a conformation-dependent epitope in the same region. Amino acid residues 23 to 239 were required and sufficient for the formation of the conformational epitope. Using these antibodies, we show that the N protein of MV is synthesized as a relatively unfolded protein which first appears in the free-protein pool. This nascent N protein undergoes a conformational change into a more folded mature form. This change does not require the participation of other viral proteins or genomic RNA. The mature N protein does not accumulate in the free-protein pool but is quickly and selectively incorporated into the viral nucleocapsids. The mature N protein is a target for interaction with the phosphoprotein (P protein) of MV. This interaction interferes with the recognition of the N protein by the N46 MAb. This suggests that the association with the P protein may mask the binding site for the N46 MAb or that it induces a conformational change in the N protein.

Sendai virus gene expression in lytically and persistently infected cells

Sendai virus RNA species were quantitated in lytically and persistently infected cultured cells by Northern blot hybridization to region- and strand-specific cloned cDNA probes. Levels of NP, P and M mRNA in lytically infected cells were equally high, but F and HN mRNA were present in about 3-fold, and L mRNA in 30-fold, lower amounts, reflecting transcriptional attenuation especially at the M-F and HN-L gene junction. Two persistently infected cell lines, which release only 1% of the virus particles of lytically infected cells, were shown to contain only 4- to 8-fold-less amounts of each viral mRNA and 2- to 3-fold-less genomic RNA than lytically infected cells. Additionally, transcription was neither defective nor more attenuated as compared to the lytical infection. Taken together the results suggest the existence of an additional regulatory mechanism for the virus release. A cell-associated state of infection therefore seems to be achievable by a relatively weak general reduction of the copy numbers of viral mRNA and genomic RNA.

Budding efficiency of Sendai virus nucleocapsids: influence of size and ends of the RNA

The budding efficiency of Sendai virus antigenomes, as well as of defective interfering (DI) nucleocapsids of the deletion and copy-back types, was compared to that of the viral genome during infections of baby hamster kidney (BHK) cells. The antigenomes were shown to bud into virus particles as efficiently as the genomes, arguing for the irrelevance of the nucleocapsid-RNA ends in regulating the efficiency of budding. The DI nucleocapsids, however, were restricted in their budding by factors inversely proportional to their size, arguing for an effect of nucleocapsid size in this process. This restriction in budding, however, appeared to be only expressed under conditions of very efficient DI-RNA replication.

A recent field isolate of Sendai virus has a temperature-sensitive HN glycoprotein

A recent field isolate of Sendai virus was found to have a temperature-sensitive (ts) hemagglutinin-neuraminidase (HN) glycoprotein. The ts phenotype was manifested as a loss of cell binding, reduced replication, and virions that were lacking surface HN after growth at the nonpermissive temperature (38 degrees). Low neuraminidase activity and failure of the field isolate to remove sialic acid from receptors on the surface of erythrocytes indicated that rapid elution of the field isolate virions from erythrocytes at the nonpermissive temperature was not due to neuraminidase activity but to a proposed conformational change in the HN molecule. The specific amino acids responsible for the ts phenotype could not be determined due to the number of amino acid differences between the field isolate and Enders strain. Heat inactivation and monoclonal antibody inhibition of HN functions indicated that the HN protein of this isolate was, in addition to ts, an unstable molecule.

Isolation of a biologically active soluble form of the hemagglutinin-neuraminidase protein of Sendai virus

As a first step in establishing the three-dimensional structure of the Sendai virus hemagglutinin-neuraminidase (HN), we have isolated and characterized a potentially crystallizable form of the molecule. The sequence of HN, a surface glycoprotein, predicts a protein with an uncharged hydrophobic region near the amino terminus which is responsible for anchorage in the viral envelope. To avoid rosette formation (aggregation), which would preclude crystallization, this hydrophobic tail was removed from a membrane-free form of HN by proteolytic digestion. This digestion resulted in a single product with a molecular weight of about 10,000 less than native HN. N-terminal amino acid sequence analysis of cleaved HN (C-HN) indicated a single cleavage site at amino acid residue 131, resulting in a product consisting of the carboxyl-terminal 444 amino acids of HN. Functional analyses revealed that C-HN retained full neuraminidase activity and was able to bind erythrocytes, indicating that the N-terminal 131 residues were not necessary for these biological activities. Furthermore, this cleavage product retained the antigenic structure of intact HN, since monoclonal antibodies still bound to C-HN in enzyme-linked immunosorbent assay and Western (immuno-) blot analysis. Viewed by electron microscopy, the dimeric and tetrameric forms of intact HN form rosettes while C-HN maintains the oligomeric structure but no longer aggregates. Furthermore, the electron micrographs revealed a C-HN tetramer strikingly similar to the influenza virus neuraminidase in both size and gross structural features.

Carboxyl-terminal region of Sendai virus P protein is required for binding to viral nucleocapsids

The Sendai virus P protein is a component of the viral nucleocapsid, where it participates in RNA synthesis. To identify domains of the protein involved in nucleocapsid recognition, deleted P protein molecules were generated from a cDNA clone of its gene. In vitro transcription of the complete gene and translation of the transcript generated a protein with electrophoretic mobility and immunoreactivity indistinguishable from those of authentic P protein. The in vitro product bound specifically to nucleocapsids when mixed with extracts from infected cells. However, a product lacking only 30 carboxyl-terminal amino acid residues (5% of the molecule) did not bind. Residues within a 195 amino acid region, adjacent to and overlapping by one amino acid with the carboxyl-terminal 30 residues, were also required for binding. No other protein region was required. Therefore, the 224-residue region which includes the carboxyl terminus appears to contain the nucleocapsid attachment site, and the 30 terminal residues either form part of the site or are required to maintain an active conformation.

Dynamics of functional maturation and inactivation of HN glycoprotein in NDV-infected chick embryo fibroblasts

In avirulent NDV strain-infected chick embryo cells treated with cycloheximide at different intervals post infection a decrease of the level of hemagglutinating (HA) and neuraminidase (Nase) activities was observed. Studies on this system led to conclusion that the HA-Nase (HN) glycoprotein molecules are unstable and the actual amount of the functionally active (mature) HN entities is determined by a dynamic equilibrium between the antidromic processes of the HN functional maturation and inactivation. Kinetic studies on the actual intracellular levels of the HA and Nase activities using 5 min intervals of their detection after the cycloheximide treatment permitted to uncouple the processes of the HN maturation and inactivation. Analytical part of the studies made it possible to compute quantitative parameters of the involved processes: (a) pool size of the functionally nonactive HN precursors, (b) time needed for their functional maturation, and (c) rate of their inactivation.

Distinct functions of antigenic sites of the HN glycoprotein of Sendai virus

Monoclonal antibodies specific for the hemagglutinin-neuraminidase (HN) glycoprotein of Sendai virus were used to examine the antigenic structure of HN and its role in the initiation of infection and immunity. Using 10 anti-HN antibodies, four distinct antigenic sites designated I-IV were topographically mapped on the HN molecule by competitive-binding assays. To relate the biological functions of HN to its antigenic structure, anti-HN antibodies were analyzed for their inhibitory activity in neuraminidase, hemagglutination, and hemolysis inhibition tests. Antibodies to antigenic site I inhibited hemagglutination and one of these antibodies also inhibited neuraminidase activity. Antibodies to site II inhibited neither activity. However, hemolysis an F protein activity was inhibited, suggesting that these antibodies which bind to HN interfere with F-mediated fusion. Antigenic sites III and IV had different effects on the hemagglutinating and neuraminidase functions of HN: Site III antibodies inhibited hemagglutination while antibodies to site IV only inhibited neuraminidase activity. Antibodies to each antigenic site inhibited virus production. Since antibodies to sites I and III inhibited hemagglutination, it is likely that they block virus adsorption. Antibodies to HN site II only inhibited hemolysis, and therefore, may prevent virus penetration. Antibodies reacting with site IV inhibited virus production after virus penetration. Since neuraminidase activity was the only function inhibited, the viral enzyme may be involved in virus release. The fact that site IV antibodies inhibited neuraminidase but not hemagglutination suggests that these sites are distinct.

Localization of P, NP, and M proteins on Sendai virus nucleocapsid using immunogold labeling

The distribution of NP, P, and M proteins on Sendai virus nucleocapsids purified from cells and virions were studied by immunogold staining using monoclonal antibodies. NP molecules were found uniformly along the entire length of both cytosol and virion derived nucleocapsids. This observation is in accord with the earlier proposals that NP molecules maintained the structural integrity of the nucleocapsid. The distribution of P in nucleocapsids derived from the cytosol differed from the distribution in those originating from virions. In nucleocapsids derived from the cytosol, P molecules occurred in 4 to 10 discreet clusters at varying locations along the length of the nucleocapsid. In contrast, on nucleocapsids derived from virions, P molecules were uniformly distributed over the entire length of the nucleocapsid. These observations suggest that the distribution of P depends on the functional state of the nucleocapsid. The occurrence of P clusters at different locations on intracellular nucleocapsids indicates that P is a mobile molecule; this suggestion is consistent with P's role in viral RNA synthesis. The distribution of the matrix (M) protein also depended on where the nucleocapsids were derived from. Large quantities of M protein were found along the entire length of nucleocapsids derived from the cytosol, while in virion nucleocapsids, many fewer molecules of M were observed. The large amounts of M on the nucleocapsids originating from the cytosol supports the hypothesis that M protein mediates the recognition between the nucleocapsid and the envelope glycoproteins.

Cytochalasin D accelerates the release of Newcastle disease virus from infected cells

The role of the cellular cytoskeleton in Newcastle disease virus (NDV) infection was explored in two ways. First, the extent of the association of viral proteins with the cytoskeletal fraction of chicken embryo cells was determined. NDV-infected cells, pulse-labelled with [35S]methionine with or without a subsequent chase, were fractionated into Triton X-100-soluble and cytoskeletal fractions. All NDV proteins become associated with the cytoskeletal fraction of cells subsequent to their synthesis. Mixing experiments provided evidence against nonspecific sticking of proteins with this cell fraction. Second, the functional significance of the cytoskeletal association was explored using the inhibitor cytochalasin D. In the presence of this inhibitor, the rate of release of radioactively labelled virions was accelerated 2.5-fold. Colchicine did not significantly alter the rate of virion release. Virus particles released from cytochalasin D-treated cells had the same density as virions released from untreated cells, but were slightly less infectious and contained less actin. These results suggest that functional microfilaments do not play an obligatory role in viral morphogenesis but rather function to slow virus particle release.

Sequence determination of the Sendai virus HN gene and its comparison to the influenza virus glycoproteins

The nucleotide sequence of the Sendai virus (SV) HN (hemagglutinin-neuraminidase) gene was determined. The deduced primary structure of the protein showed only one hydrophobic domain likely to represent the transmembrane region, but at its N terminus. Since the SV F protein is anchored in the membrane at its C terminus, the two SV glycoproteins are thus membrane-anchored in opposite orientations, similar to the two influenza virus (FLU) glycoproteins. Amino acid sequence comparisons of the SV HN and the FLU HA and NA proteins revealed homologies between 100 amino acids of the hemagglutinin region of the FLU HA protein and the C terminus of the SV HN, and between 200 amino acids of the neuraminidase region of the FLU NA and the central region of SV HN. Alignment of the neuraminidase, hemagglutinin, and fusion regions shared by these glycoproteins suggest the structure of a possible ancestral gene.

An alternative route of infection for viruses: entry by means of the asialoglycoprotein receptor of a Sendai virus mutant lacking its attachment protein

During the first stage of infection, the paramyxovirus Sendai virus attaches to host cells by recognizing specific receptors on the cell surface. Productive virus-cell interactions result in membrane fusion between the viral envelope and the cell surface membrane. It has recently been shown that the ganglioside GD1a and its more complex homologs GT1b and GQ1b are cell surface receptors for Sendai virus. We report in this paper that the temperature-sensitive mutant ts271 of the Enders strain of Sendai virus lacks the viral attachment protein HN and the biological activities of hemagglutination and sialidase activity associated with it when the virus is grown at 38 degrees C. This HN- virus was unable to infect or agglutinate conventional host cells that contained receptor gangliosides and were readily infected by the parental wild-type virus. The HN- virus did, however, attach to and infect Hep G2 cells, a line of hepatoma cells that retains the asialoglycoprotein receptor (ASGP-R) upon continuous culture. This receptor is a mammalian lectin that recognizes galactose- or N-acetylgalactosamine-terminated proteins. In accordance with the known properties of this receptor, infection by the HN- virus was abolished by treatment of Hep G2 cells with sialidase, by the presence of Ca2+ chelators, and by competition with N-acetylgalactosamine, asialoorosomucoid, and antibody to the receptor. F, the only glycoprotein on the HN- virus, was shown to compete with the galactose-terminated protein asialoorosomucoid for the ASGP-R. The ability of the HN- virus to cause cell-cell fusion of Hep G2 cells indicated that attachment of this virus to the ASGP-R still permitted viral entry by its usual mode--i.e., membrane fusion at the cell surface. These results open up the possibility that enveloped viruses, which contain glycosylated proteins or lipids, may make use of naturally occurring lectins in addition to their normal receptors as a means of attachment to host cells.

Monoclonal antibodies to the P protein of Sendai virus define its structure and role in transcription

Four monoclonal antibodies specific for Sendai virus nucleocapsid protein P were used to examine both the antigenic structure of P and its role in transcription. Three distinct antigenic regions were delineated on P by competitive radioimmunoassays (RIAs), and through Western blot analysis all three sites were mapped to a 40,000-MW (40K) Staphylococcus aureus protease V8-digestion fragment, which remains associated with the neucleocapsid structure. To study the function of P, nucleocapsids were treated with saturating amounts of anti-P monoclonal antibodies and it was found that transcription in vitro was inhibited by 60-90%. Data, therefore, are consistent with the conclusion that the P protein is required for transcription and that the 40K protease-resistant core contains the functionally important portion of the molecule. Further analysis of the P structure showed that some of the 40K fragments were linked by disulfide bonds. These results suggest that the protease-resistant 40K fragment is in the carboxyl-terminal half of P, since the three cysteine residues of P are found there (C. Giorgi, B. M. Blumberg, and D. Kolakofsky (1983), Cell 35, 829-836).

Structural and functional analysis of Sendai virus nucleocapsid protein NP with monoclonal antibodies

Monoclonal antibodies specific for Sendai virus nucleocapsid protein NP were used to map the antigenic structure of NP and to investigate the role of NP in transcription. Using nine anti-NP antibodies in competitive-binding (CB) assays, it was found that the NP molecule contained at least two topographically distinct antigenic sites. By Western blot analysis, one of the NP epitopes belonging to antigenic site I was localized to a Mr 34,000 (34K) trypsin digest fragment, and another to a Mr 48,000 (48K) fragment which remained associated with the nucleocapsid. The other antibodies which define antigenic site I did not react with either fragment; however, the results of CB would indicate that their epitopes were in a region on the tertiary structure of the NP molecule that is closely proximal to these fragments. The 48K and 34K fragments on the published NP amino acid sequence have been tentatively identified. Since the 34K and 48K fragments bind antibody, it appears that nucleocapsid-bound NP may be folded into a configuration which places at least some of these sequences on the surface of the nucleocapsid structure. Six antibodies representing both antigenic sites were purified for functional studies. All the antibodies inhibited nucleocapsid transcription in vitro to the same extent (greater than 90%); however, they differed in the amount of antibody required to produce the same effect. Within site I, antibodies producing maximum inhibition were divided into three groups: three antibodies inhibited at relatively low concentrations (0.17 microgram), one antibody inhibited at an intermediate range (0.43 microgram), and another required a 10-fold higher concentration (1.73 microgram) to produce the same effect. The antibody which detected the 48K trypsin digest fragment was the one which fell into the intermediate range for transcription inhibition, while the antibody that detected the 34K fragment was in the low range. Thus, antigenic site I, defined by CB and trypsin digestion studies, can be defined further into three subsites which appear to differ in their involvement in the transcription process. Antigenic site II was defined by a single antibody which also inhibited transcription by greater than 90%.

Restriction of cell surface expression of Sendai virus hemagglutinin-neuraminidase glycoprotein correlates with its higher instability in persistently and standard plus defective interfering virus infected BHK-21 cells

To gain an understanding of the mechanism(s) by which Sendai virus generates a persistent infection, the expression of the hemagglutinin-neuraminidase (HN) and fusion (Fo) glycoproteins at the surfaces of BHK-21 cells infected with standard virus, a mixture of standard and defective interfering (DI) particles (mixed virus infection), and during persistent infection was investigated. The expression of HN and Fo was measured on the surfaces of infected cells by the binding of anti-HN and anti-Fo monoclonal antibodies. The results show that HN expression was restricted relative to Fo during mixed virus and persistent infections. The decreased levels of HN were investigated further by pulse-chase experiments which revealed that HN has an increased turnover rate in persistently infected cells and, to a lesser extent, in mixed virus infected cells. In analyzing the [35S]methionine-labeled protein composition of virus particles produced during the pulse-chase experiments, the increased turnover of newly synthesized HN was found to correlate with its decreased incorporation into virus particles. Interestingly, the poor HN incorporation also correlates with less efficient incorporation of the matrix M protein into virus particles.

Evidence for an interaction between the membrane protein of a paramyxovirus and actin

Evidence for an interaction of the membrane (M) protein of Newcastle disease and Sendai viruses with cellular actin was obtained by three different techniques. M protein linked to Sepharose 4B was found to bind actin, but not myoglobin or bovine serum albumin, and to selectively remove actin from a mixture of these three proteins. Sedimentation of a mixture of M protein and F-actin through a sucrose gradient resulted in sedimentation of M protein with actin. Control proteins, bovine serum albumin and cytochrome c, did not sediment with actin. In circular dichroism studies, M protein added to actin in a 1:1 complex resulted in a significant increase in negative ellipticity at 220 nm, which corresponds to an increase in alpha-helix and a decrease in beta-structure and random coil. This is indicative of an interaction between M protein and actin. It is possible that the frequent identification of cellular actin in a number of enveloped viruses may be attributed to the interaction of actin and M protein or its equivalent.

Restriction of virus-specific protein synthesis in a persistent paramyxovirus infection

Synthesis of virus-specific proteins in a persistent, nonproductive paramyxovirus infection derived from the peripheral blood leukocytes of a patient with subacute sclerosing panencephalitis (SSPE) was investigated. The persistently infected cells expressed cytoplasmic virus-specific antigens and generated paramyxovirus nucleocapsids throughout long-term passage. When analyzed by specific immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, most of the virus structural proteins were synthesized in acutely infected cells, but only three of the proteins could be readily detected in persistently infected cells. The two structural proteins whose synthesis was most clearly restricted had molecular weights of 69,000 and 41,000 daltons and represented the putative HN and M virus proteins. The similarities between the restriction of virus protein synthesis in this system and that reported previously for other persistent paramyxovirus infections derived from SSPE suggest that a common mechanism may be involved in the maintenance of such infections.

Assembly of viral structural proteins in cells infected with a temperature-sensitive mutant derived from an HVJ (Sendai virus) carrier culture. Brief report

The processing of virus polypeptides synthesized in cells infected with HVJ (haemagglutinating virus of Japan--the Sendai strain of parainfluenza 1 virus) was studied. Maturation of a temperature-sensitive (ts) mutant (HVJ-pB) derived from an HVJ carrier culture was inhibited at 38 degrees C incubation. A considerable amount of viral components were made at the restrictive temperature. They were, with the exception of the polypeptide HN, well preserved without a great loss of their function and successfully incorporated into virus particles released after lowering the incubation temperature. The membrane (M) protein seems to be essential for virus morphogenesis.

Measles virus polypeptides in purified virions and in infected cells

A wild-type measles virus was radiolabeled during growth in VERO cells and purified by two successive potassium tartrate gradient centrifugations. The virion polypeptide composition was determined by SDS-polyacrylamide gel electrophoresis employing two different buffer systems. Six virus-specific polypeptides were consistently detected. The largest (L) had a molecular weight (MW) of greater than 150,000. The second largest polypeptide, G (MW 79,000), was the only glycoprotein found. The proteins designated polypeptide 2 (MW 66 to 70,000) and nucleocapsid protein or NP (MW 61,000) were phosphorylated. The remaining virus-coded proteins were polypeptide 5 (MW 40,000) and the matrix or M protein (MW 37,000). Measles virions also contained a polypeptide (MW 42,000) thought to be actin due to co-migration with this component of uninfected cells. Analysis of in vitro 3H-acetic anhydride radiolabeled virions confirmed the presence of these seven polypeptides. Acetic anhydride also labeled a protein designated polypeptide 4 (MW 53,000) which was not consistently radiolabeled in vivo, as well as several other minor proteins believed to be cellular in origin. Synthesis of the six virus-specific structural polypeptides was detected in lysates of infected cells by SDS-polyacrylamide slab gel electrophoresis. Virus specificity of polypeptide 4 could not be confirmed due to the similar MW of several cellular polypeptides. Two non-virion, but virus-specified polypeptides, of MW 38,000 and 18,000 were also detected. Synthesis of the virus structural proteins was in the same proportions as the polypeptides found in virions except for under production of polypeptide G and over production of polypeptide 2.

Gycoprotein and protein precursors to plasma membranes in vesicular stomatitis virus infected HeLa cells

Vesicular stomatitis virus is known to mature at HeLa cell plasma membranes. To study the process, cells, infected with vesicular stomatitis virus, were fractionated after short term labeling studies (1 min pulse, 1 min chase) to determine the assembly kinetics of G protein and M protein into plasma membranes. Newly synthesized M protein was found released in the supernatant from which free polysomes were sedimented during sucrose gradient analysis of these polysomes. If this M protein is particle bound, it must have a density of less than 1.08 g/ml. About 40% of this M protein so labeled was not sedimentable at 165,000 X g for 16 h. This newly synthesized M protein had not yet assembled into plasma membrane and thus must represent an internal pool. This and previous studies show that it has a subsequent transit time to the plasma membrane of about 2 min. Once associated with plasma membranes, M protein decayed in an approximately logarithmic fashion indicating that newly synthesized M randomly mixes (and turns over) with preexisting M protein. G protein was particle bound in a 1 min pulse, 1 min chase, and was never found released in a soluble form. At the later time when fucose is added to G protein, the oligosaccharide moiety is near to complete, and on completion is about 2,000 in molecular weight. Evidence is presented showing that fucose is probably attached to the N-acetylglucosamine of the protein carbohydrate linkage. G protein to which fucose had just been added was located internally on a membranous fraction of density 1.14 g/ml in sucrose; its subsequent transit time from this pool (which in uninfected cells is between 1--2% of the total cell fucosyl glycoprotein) was about 15 min. Because their densities were different and their transit times were different, internal newly synthesized M and fucosyl G protein which assemble into plasma membranes were not on the same internal membranous component. Association of M protein with the plasma membranes may thus occur from a nonsedimentable soluble cytoplasmic pool by a process of direct adsorption.