Structural Studies of Macromolecular Assemblies 
Stephen C. Harrison, Ph.D. — Investigator 
Dr. Harrison is also Professor of Biochemistry and Molecular Biology at Harvard University and Research 
Associate in Medicine at the Children's Hospital, Boston. He received his A.B. degree in chemistry and 
physics from Harvard College and his Ph.D. degree in biophysics from Harvard University. Dr. Harrison 
was recently elected to the National Academy of Sciences. 
HOW do viruses enter and leave cells? How do 
receptors and their ligands cycle from cell 
surface to cell interior and back? How do regula- 
tory proteins activate or inhibit transcription of 
particular genes? These questions deal with mo- 
lecular recognition in the determination of cell 
organization. They represent groups of projects 
in our laboratory, all of which involve elucida- 
tion of detailed atomic structures as a prerequi- 
site for tackling functional problems. 
Viruses 
The small double-stranded DNA viruses SV40 
and polyoma have given us our first glimpse of 
virus particles that package a minichromosome in 
one cell and deliver it to the nucleus in another. 
The shells of these viruses are composed of 72 
pentamers of the major structural protein VPl 
and 30-60 copies each of two internal proteins, 
VP2 and VP3. These components package the 
viral DNA. The VPl polypeptide chain folds in 
such a way that two large |8-sheets with radially 
directed strands form a framework, with tight in- 
teractions between adjacent subunits in a pen- 
tamer. The carboxyl terminus of VPl forms an 
extended arm that interacts with subunits of an- 
other pentamer, generating three kinds of inter- 
pentamer contact in the virus particle. Flexibly 
extended arms, which form ordered structures 
only when the units assemble into a particle, ap- 
pear to be an important feature of complex 
assemblies. 
Comparison of a low-resolution structure of 
polyoma with the SV40 model shows that two 
surface loops are larger in the polyoma subunit. 
Mutational evidence suggests that these loops 
create a shallow pocket for binding sialic acid, 
required for cell entry by polyoma but not by 
SV40. A number of viruses of various structural 
types use cell-surface sialic acid for attachment, 
and it is of broad interest to understand how spe- 
cific viruses accomplish the interaction. Compar- 
ison of SV40 and polyoma shows that this func- 
tion can readily be added or lost by small changes 
at the surface of a viral coat protein. We have 
prepared cocrystals of polyoma with bound sialyl 
lactose in order to confirm assignment of its site 
and to study the molecular details. 
We have recently begun to study the double- 
stranded RNA viruses. Crystals of rotavirus single- 
shelled particles and reovirus cores diffract to at 
least 7 A resolution. These particles are elabo- 
rately organized molecular machines, containing 
the complete transcription and RNA-modifica- 
tion activities. 
Receptors 
The receptor for human immunodeficiency 
virus (HIV) is the lymphocyte surface antigen 
CD4. Its extracellular portion is composed of 
four immunoglobulin-like domains. We have de- 
termined the atomic structure of a two-domain 
amino-terminal fragment, which binds HIV as 
tightly as does the intact receptor. The first two 
domains are formed by a continuous |S-strand 
connector, and they have an extensive hydropho- 
bic interface. Thus they form a rigid rod-like seg- 
ment. The HlV-binding site appears to be a ridge 
along one edge of the first domain. 
We have also crystallized a rather different sort 
of cell-surface protein — the extracellular do- 
main of the human transferrin receptor. This mol- 
ecule undergoes a well-characterized cycle of 
uptake and return to the cell surface. The exter- 
nal domain, which makes up about three-fourths 
of the molecule, exhibits reversible conforma- 
tional changes at low pH that we believe to be 
signals for intracellular sorting steps. 
Transcriptional Regulatory Complexes 
A common characteristic of eukaryotic tran- 
scriptional regulatory elements is the presence of 
sites in multiple copies that vary slightly in se- 
quence, often with two or more related proteins 
that can bind to them. The best understood pro- 
karyotic paradigm is in the immunity region of 
temperate bacteriophages, where two proteins, 
repressor and Cro, bind two sets of three sites, 
with appropriately graded affinities. We have 
made an effort to understand the mechanism of 
this regulatory switch, by determining the struc- 
tures of a series of specific protein/DNA com- 
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