per, a two-stranded, parallel coiled coil. Science 
254:539-544. 
O'Shea, E.K., Rutkowski, R., and Kim, P.S. 1992. 
Mechanism of specificity in the Fos-Jun oncopro- 
tein heterodimer. Ce// 68:699-708. 
Staley, J.P., and Kim, P.S. 1992. Complete folding 
of bovine pancreatic trypsin inhibitor with only a 
single disulfide bond. Proc Natl Acad Set USA 
89:1519-1523. 
Talanian, R.V., McKnight, C.J., Rutkowski, R., and 
Kim, P.S. 1992. Minimum length of a sequence- 
specific DNA binding peptide. Biochemistry 
31:6871-6875. 
Weissman, J.S., and Kim, P.S. 1991. Reexamination 
of the folding of BPTI: predominance of native 
intermediates. Science 253:1386-1393. 
Weissman, J.S., and Kim, P.S. 1992. The disulfide 
folding pathway of BPTI: response. Science 
256:112-114. 
GENETICS AND BIOCHEMISTRY OF RNA VIRUSES 
Karla A. KiRKEGAARD, Ph.D., Assistant Investigator 
RNA virus and virus-like genomes replicate in 
cells of bacteria, fungi, plants, and animals. Many 
proteins and subcellular structures from the host 
cells are undoubtedly used by the viruses during 
their replicative cycles, but only a few of these host 
components have been identified. For example, Q/? 
phage utilizes translation factors from its Escher- 
ichia coli host as subunits of its RNA replicase, en- 
coding only the catalytic subunit with its own ge- 
nome. Poliovirus, a positive-strand RNA virus, can 
specifically infect primate cells because the polio- 
virus receptor, a cellular adhesion protein identi- 
fied in the laboratory of Dr. Vincent Racaniello (Co- 
lumbia University), is present on surfaces of these 
host cells. During the 7-h course of a lytic infection 
with poliovirus, numerous changes occur in the 
host cells: cellular translation is inhibited, tran- 
scription is depressed, and membranous vesicles 
with which the viral replication machinery is specif- 
ically associated accumulate. Dr. Kirkegaard's labo- 
ratory is interested in identifying proteins in mam- 
malian cells employed during poliovirus infection 
by assaying their altered function during infection, 
their specific binding to poliovirus proteins, and 
any changes in these properties during infection 
with well-characterized mutant polioviruses. 
The budding yeast Saccharomyces cerevisiae, in 
which virus-like particles containing double- 
stranded RNA genomes replicate in large numbers, 
provides a system in which the contributions of host 
proteins to RNA viral replication can be assessed ge- 
netically. As many as 1 00,000 copies per cell of one 
such virus, L A, are present in most laboratory strains 
of S. cerevisiae. Unlike lytic viruses, the presence of 
L-A particles is apparently not detrimental to yeast 
cells. Dr. Kirkegaard's laboratory has found that 
under certain conditions the L A virus particles pro- 
vide a growth advantage to its host, suggesting a mu- 
tualistic relationship between the RNA genome and 
the yeast cell. 
Inhibition of Protein Secretion in Host 
Cells During Poliovirus Infection 
The origin of the membranous vesicles that accu- 
mulate during poliovirus infection of primate cells 
and the mechanism of their formation are unknown. 
However, recent findings from the Kirkegaard labo- 
ratory argue that these vesicles are derived from the 
normal protein secretion apparatus of the infected 
cell, either from the endoplasmic reticulum (ER) or 
the cis compartment of the Golgi apparatus. First, 
the surprising observation that brefeldin A, an inhib- 
itor of ER-to-Golgi transport, specifically inhibited 
poliovirus RNA synthesis suggested that poliovirus 
RNA replication requires normal ER and Golgi func- 
tion. However, poliovirus encodes no glycosylated 
or secreted proteins; it is thus possible that the func- 
tion required from the host ER or Golgi might be the 
vesicle formation through which normal protein 
traffic occurs. 
Two recent sets of experiments have supported 
this model. In brefeldin A-resistant cell lines, polio- 
virus replication was found to be brefeldin A resis- 
tant; therefore it is a cellular function that mediates 
the inhibition observed in wild-type cells. Further- 
more, when VSV G, a membrane protein used to 
follow the glycosylation events that accompany nor- 
mal protein secretion, was expressed in poliovirus- 
infected cells, Dr. Kirkegaard and her colleagues 
found that its transit through the protein secretion 
apparatus halted between the ER and the medial 
Golgi compartment. Currently they are testing 
whether the vesicles that accumulate during polio- 
virus infection bear ER or cis-Golgi markers and if 
CELL BIOLOGY AND REGULATION 79 
