oped a spectrofluorimctric method designed to 
allow simultaneous illumination and excitation- 
emission fluorescence measurements of mixtures of 
recombinant proteins. Rhodopsin-catalyzed binding 
of GTP or a GTP analogue to transducin results in a 
large increase in its intrinsic fluorescence. Mixtures 
of transducin and rhodopsin can be assayed by this 
method to determine the kinetic rate constants of 
their interaction and to evaluate the specific effects 
of mutations. 
Studies of a series of site-directed mutants of rho- 
dopsin with alterations in their cytoplasmic do- 
mains have been performed. The eventual aim of 
this approach is to reconstitute purified recombi- 
nant rhodopsin and transducin under defined condi- 
tions where specific physical measurements relat- 
ing to their interaction can be obtained. 
Dr. Sakmar is also Assistant Professor and Head 
of Laboratory at the Rockefeller University. 
Books and Chapters of Books 
Sakmar, T.P., Franke, R.R., and Khorana, H.G. 
1992. Mutagenesis studies of rhodopsin photo- 
transduction. In Signal Transduction in Photo- 
receptor Cells (Hargrave, P.A., Hofmann, K.P., 
and Kaupp, U.B., Eds.). Berlin: Springer-Verlag, 
pp 21-30. 
Articles 
Chan, T., Lee, M., and Sakmar, T.P. 1992. Intro- 
duction of hydroxyl-bearing amino acids causes 
bathochromic spectral shifts in rhodopsin. Amino 
acid substitutions responsible for red-green color 
pigment spectral tuning. /5/o/ Chem 267:9478- 
9480. 
Franke, R.R., Sakmar, T.P., Graham, R.M., and 
Khorana, H.G. 1992. Structure and function in 
rhodopsin. Studies of the interaction between the 
rhodopsin cytoplasmic domain and transducin. / 
Biol Chem 267:14767-14774. 
Lin, S.W., Sakmar, T.P., Franke, R.R., Khorana, 
H.G., and Mathies, R.A. 1992. Resonance Raman 
microprobe spectroscopy of rhodopsin mutants: 
eff'ect of substitutions in the third transmembrane 
helix. Biochemistry 31:5105-51 1 1. 
Sakmar, T.P. 1992. The traveler's medical kit. In- 
fect Dis Clin North Am 6:355-370. 
DEVELOPMENT AND FUNCTION OF THE SYNAPSE 
Richard H. Scheller, Ph.D., Associate Investigator 
The nervous system is composed of large numbers 
of unique cells that communicate with each other 
via the regulated release of chemical neurotransmit- 
ters. These synaptic interactions govern animal be- 
havior. Modulation of the efficacy of synaptic com- 
munication is thought to underlie learning and 
memory. Dr. Scheller and his colleagues are inter- 
ested in understanding the molecular mechanisms 
of synaptic formation during development and re- 
generation in the peripheral nervous system after 
nerve injury. It is also a goal to contribute to an 
understanding of how the nerve terminal functions 
in regulating release of neurotransmitters. 
Synapse Development 
Motor neurons in the spinal cord send axons to 
muscle fibers throughout the body. When axons 
contact muscle fibers, a highly ordered structure 
consisting of a presynaptic nerve terminal and post- 
synaptic site develops. The postsynaptic element is 
made up of a membrane rich in receptors for the 
neurotransmitter and an indentation in the mem- 
brane called the junctional fold. One key event in 
the development of the neuromuscular junction is 
the redistribution of neurotransmitter receptors that 
occurs when nerve contacts muscle. Initially recep- 
tors for the neurotransmitter, in this case acetylcho- 
line, are randomly distributed on the muscle fiber. 
When the nerve contacts muscle, neurotransmitter 
receptors aggregate underneath the nerve terminal 
in an appropriate position to detect the chemicals 
released during synaptic transmission. 
Agrin is a component of the extracellular matrix that 
causes acetylcholine receptors to cluster when added 
to muscle fibers growing in culture. Dr. Scheller and 
his colleagues have isolated recombinant DNA clones 
encoding agrin molecules and through an analysis of 
the nucleotide sequence defined the primary amino 
acid sequence of the molecule. Comparison of the pre- 
dicted agrin sequence to the proteins in the data bank 
revealed similarities to two domains of laminin, kazal- 
type protease inhibitors, epidermal growth factor 
(EGF) repeats, serine/threonine-rich regions, and in- 
ternal repeats. Analysis of the genomic sequence dem- 
NEUROSCIENCE 433 
