are available for some. Dr. Hynes and his colleagues 
have identified alternatively spliced forms of inte- 
grins, as well as novel integrins in Drosophila, and 
are analyzing their expression and function. Using 
the available mutations, the laboratory has demon- 
strated roles for certain integrins in the develop- 
ment of early embryos, muscles, wings, and eyes. By 
using P-element-mediated transformation to intro- 
duce integrin genes into the fly genome, the mutant 
phenotypes can be rescued, and, by appropriate con- 
structions, the times at which integrins are required 
and the roles of the different spliced forms have 
been investigated. Site-specific mutagenesis is being 
used to analyze the roles of specific segments of the 
integrin subunits. In particular, the roles of the cy- 
toplasmic domains thought to be involved in cyto- 
skeletal connections are under study. 
The functions of the cytoplasmic domains are also 
being examined by transfection of mutated integrin 
subunit genes into mammalian cells. These experi- 
ments implicate the cytoplasmic domain of the /3, 
subunit in association with the cytoskeleton. Similar 
studies are being initiated on the a subunits, particu- 
larly three related integrins («3i8i, a^^-^) that 
are all receptors for fibronectin, albeit for difl'erent 
regions, including, for a^^^, one of the alternatively 
spliced segments. 
In parallel with these studies on integrins, talin, 
one of the cytoskeletal proteins thought to interact 
with integrins, has been cloned and sequenced. This 
has allowed definition of its overall structure and 
the homology of one domain with other membrane- 
associated cytoskeletal proteins. These clones are 
being used to determine the binding sites for poten- 
tial interacting proteins, including integrins and 
vinculin, another cytoskeletal protein involved in 
connections between integrins and actin. The aim is 
to elucidate the molecular structure of the "focal" 
contact, where extracellular matrix is connected to 
the cytoskeleton via integrins. Focal contact organi- 
zation and function are altered in tumor cells. Thus 
it is an important point of regulation of cell 
behavior. 
The work on in vitro expression of integrins and 
fibronectins and some of the research on mice is 
supported by grants from the National Cancer Insti- 
tute and the National Heart, Lung and Blood 
Institute. 
Dr. Hynes is also Professor of Biology at the 
Massachusetts Institute of Technology. 
Articles 
DeSimone, D.W., Norton, P.A., and Hynes, R.O. 
1992. Identification and characterization of alter- 
natively spliced fibronectin mRNAs expressed in 
C2LT\y Xenopus embryos. Dev Biol 149:357-369. 
Guan, J.-L., Trevithick, J.E., and Hynes, R.O. 1991. 
Fibronectin/integrin interaction induces tyrosine 
phosphorylation of a 120 kDa protein. Cell Regul 
2:951-964. 
Hynes, R.O. 1992. Integrins: versatility, modula- 
tion and signaling in cell adhesion. Cell 69:11- 
25. 
Hynes, R.O. 1992. Specificity of cell adhesion in 
development: the cadherin superfamily. Curr 
Opin Genet Dev 2:621-624. 
Hynes, R.O., and Lander, A.D. 1992. Contact and 
adhesive specificities in the associations, migra- 
tions, and targeting of cells and axons. Cell 
68:303-322. 
Salomon, D., Ayalon, O., Patel-King, R., Hynes, 
R.O., and Geiger, B. 1992. Extrajunctional distri- 
bution of N-cadherin in cultured human endothe- 
lial cells. J Cell Sci 102:7-17. 
FACTORS THAT PROMOTE HOST CELL ENTRY AND GROWTH 
OF INTRACELLULAR BACTERIA 
Ralph R. Isberg, Ph.D., Assistant Investigator 
Dr. Isberg and his colleagues are investigating the 
interaction of intracellular bacterial pathogens with 
human cells. The main concern of the laboratory is to 
determine how some pathogenic microorganisms are 
able to penetrate into host cells that are not normally 
considered phagocytic and how bacteria are able to 
evade intracellular killing mechanisms and thrive 
within host cells. To dissect these processes. Dr. 
Isberg's laboratory has concentrated on two gram- 
negative bacteria that are either efficient at entry into 
host cells or grow well within the normally hostile 
environment of phagocytes. Bacterial uptake into cul- 
tured mammalian cells is being studied with the bacte- 
rium Yersinia pseudotuberculosis, while Legionella 
pneumophila is being studied as a model for intracel- 
lular growth within macrophages. 
CELL BIOLOGY AND REGULATION 
73 
