The post-translational processing of Semliki forest virus structural polypeptides in puromycin treated cells

Posttranslational processing of Uukuniemi virus glycoproteins G1 and G2

Uukuniemi virus, which matures specifically in the Golgi complex, contains two species of envelope glycoproteins, G1 (Mr, 70,000) and G2 (Mr, 65,000). These are translated as a polyprotein, p110, from an mRNA which is complementary to the medium-sized segment of the virion RNAs. By synchronized initiation of protein synthesis and pulse-labeling, it was shown that glycoprotein G1 is amino terminal in precursor protein p110. Apparently, the nonglycosylated forms of these proteins (Mr, 54,000 to 57,000), synthesized in the presence of tunicamycin, comigrate in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, because a similar-sized protein could be isolated by immunoprecipitation with monoclonal antibodies directed against either G1 or G2. The G1 protein, which in the virion contains exclusively endoglycosidase H (endo H)-resistant glycans, was converted to the endo H-resistant form in a half time of about 45 min. The G2 protein, which in the virion has a heterogeneous glycosylation pattern as revealed by endo H digestion, attained this partial endo H resistance only after 90 to 150 min of chase. The transport time of Uukuniemi virus glycoproteins from the endoplasmic reticulum to the Golgi complex was considerably longer than that for alpha and rhabdovirus glycoproteins. Determination of the transport time of G1 and G2 to extracellular virions revealed that G1 is incorporated into mature virions about 10 min faster than G2, suggesting that G1 and G2 are transported with different kinetics to the site of virus maturation.

Correlation of glycosylation forms with position in amino acid sequence

We surveyed published reports on about 50 glycoproteins whose amino acid sequence, glycosylation sites, and type of glycosylation at a particular site have been established. We note that high-mannose substances were rarely found at the N-terminal side of a previously glycosylated complex site. There was a very definite distribution of complex sites about the N-terminal region. Furthermore, secreted glycoproteins usually contained only complex oligosaccharides whereas membrane proteins contained both types. We suggest that the position of the glycosylation site with respect to the N-terminus affects the extent of oligosaccharide processing and subsequent presentation of complex or high-mannose structures in the mature glycoprotein. This review relates glycosylation type to its position in the known sequence of given proteins and discusses these observations in light of known glycosylation processing reactions.

Evidence for a separate signal sequence for the carboxy-terminal envelope glycoprotein E1 of Semliki forest virus

When Semliki Forest virus temperature-sensitive mutant ts-3 was grown at the restrictive temperature an aberrant nascent cleavage of the 130,000-dalton structural polyprotein took place relatively frequently. This cleavage yielded an abnormal 86,000-dalton fusion protein (p86) consisting of the amino-terminal capsid protein linked to the amino acid sequences of envelope protein p62 (a precursor of E3 and E2). The other cleavage product was the carboxy-terminal envelope protein E1. p86 was not glycosylated and was sensitive to the action of protease in the microsomal fraction, whereas E1 was glycosylated and protected from proteases, indicating that it had been segregated into the cysternal side of the microsomal vesicles. All attempts to show the E1 protein at the cell surface have failed so far, suggesting that it remains associated with intracellular membranes. When ts-3-infected cells labeled at the restrictive temperature were shifted to the permissive temperature the only labeled protein released with the virus particles was E1, indicating that E1, synthesized at the restrictive temperature, was competent to participate in the virus assembly. These results suggest strongly that there are two separate signal sequences for the envelope proteins of Semliki Forest virus. One follows the capsid protein as shown previously, and the other is for the carboxy-terminal E1. Even if the insertion of the amino-terminal envelope protein (p62) fails due to a cleavage defect, the other signal sequence can operate independently to guide the E1 through the endoplasmic reticulum membrane.

Control of protein synthesis in Semliki forest virus-infected cells

Protein synthesis in Semliki forest virus-infected chicken embryo cells was studied by labeling them with [35S]methionine for short periods at different times after infection, with or without synchronization of protein synthesis by the hypertonic block technique. The rate of host-cell protein synthesis declined almost linearly in inverse correlation to the increase in the amount of virus specific RNA. At 5.5 h postinfection, the host-cell protein synthesis was reduced by about 70%. The viral structural proteins were detectable with certainty at 3.5 h postinfection, and their rate of synthesis increased linearly parallel to the amount of their messenger, the 26S RNA. This suggests that the rate of synthesis of the structural proteins is controlled at the level of transcription. The rate of synthesis of the virus-specific nonstructural proteins attained its maximum between 3 and 4 h postinfection and declined thereafter, wheras the amount of their messenger, the 42S RNA, continued to increase linearly in the cells. Thus, the messenger activity of the 42S RNA is reduced in the late phase of infection compared with its activity in the early phase.