In vitro insertions and deletions in the G segment of phage mu DNA do not abolish the inversion process

The growth of rabies virus at a low pH of the culture medium resulted in the production of a large number of spikeless virions which lacked glycoprotein protruding from the viral surface. These virions were first evident at a pH below 7.0 and reached a maximum at pH 6.7 to 6.8. The spikeless virion exhibits a lower sedimentation velocity than the infectious virion, but is similar to the infectious virion in shape and size. Our present working hypothesis is that these spikeless virions are formed by proteolytic digestion of a large portion of glycoprotein after the assembly of standard virions at the cellular membrane. This is based on the following findings: (1) Electron micrographs of infected cells in which two-thirds of the progeny virions belonged to the spikeless type revealed that spikes were observed on most cell-associated virions; (2) SDS-PAGE of the spikeless virions showed a new low molecular weight polypeptide, and a polypeptide showing a similar mobility was also detected in the infectious virions in which spikes had been cleaved off by Pronase treatment; and (3) peptide mappings of both low molecular weight polypeptides revealed similar findings. The significance of the low pH of the culture medium in producing spikeless virus is under investigation.