Molecular Genetics of Intracellular Microorganisms 
Our investigation of invasin/integrin has led to 
a model for Yersinia uptake into host cells. Bind- 
ing of invasin to its integrin receptor leads to rear- 
rangement of the cytoskeleton — rearrangement 
requisite to entry. A signal must be sent to cause 
the host cell to internalize the microorganism, 
and the internalization is facilitated by the ex- 
traordinary avidity with vi'hich invasin binds its 
receptors. Other proteins that bind the identical 
integrins cannot produce this signal so effi- 
ciently, because they do not bind the receptors 
tightly. 
So, invasin appears to promote entry of the mi- 
croorganism because it binds an important recep- 
tor that communicates with the cell cytoskeleton, 
and because it binds so tightly to this receptor. 
Legionella pneumophila Growth in 
Phagocytic Cells 
L. pneumophila causes a variety of diseases in 
humans, including Legionnaire's disease pneumo- 
nia. The bacterium grows in lung tissues after en- 
counter with its human host. Its favorite habitat is 
within alveolar macrophages, cells that normally 
function to kill invading microorganisms. An im- 
portant mechanism for macrophages to kill or in- 
hibit the growth of a microorganism is to interna- 
lize it and sequester it in a compartment called a 
phagosome, which in turn fuses with a lysosomal 
compartment filled with antibacterial factors. L. 
pneumophila is able to grow within the phago- 
some, convert it into an organelle with a unique 
morphology, and prevent the introduction of the 
antibacterial lysosomal components into this site. 
We have been interested in determining how L. 
pneumophila is able to establish and grow within 
this protective niche. Our approach has been to 
isolate mutations in this bacterium that prevent it 
from growing intracellularly. Three easily distin- 
guishable classes of mutants have been isolated. 
The first class causes the bacterium to be internal- 
ized by a macrophage via a novel pathway, and 
this causes an extreme defect in bacterial growth. 
The second class, and most easily isolated, con- 
sists of mutants that are no longer able to prevent 
the lysosomal contents from being introduced 
into the phagosome. The third class appears nor- 
mal for uptake as well as for shutting out the lyso- 
somal components, but the phagosome contain- 
ing the mutant microorganism no longer exhibits 
the unique morphology usually found in a Le- 
gionella infection. 
These classes of mutants indicate that the mi- 
croorganism performs a distinct series of steps 
within the macrophage, each of which contrib- 
utes to the parasite's efficient growth. We are 
currently trying to identify the factors missing in 
these mutants, using a combination of molecular 
and genetic techniques, in hopes of determining 
how the bacterium is able to perform each of 
these self-serving steps. 
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