Fluorescein-labeled antibody to rabbit pulmonary angiotensin-converting enzyme localized in the vascular endothelium of rabbit lung, liver, adrenal cortex, pancreas, kidney, and spleen. Epithelial cells of the renal proximal tubules were the only parenchymal cells among the organs studied that demonstrated immunoreactivity.

Extensive chemical and immunological studies have characterized the group A streptococcal cell wall as composed of four major components: M protein (and related T and R antigens), carbohydrate, mucopeptide, and glycerol teichoic acid (1, 2). These cell wall constituents, along with an external capsule and an underlying cytoplasmic membrane, are important in determining the relationship between group A streptococci and their human hosts (3). Our aim is to define the morphology and location of each of the major components of the streptococcal cell wall. The first component we have studied is the M antigen or M protein, the topic of this report.M protein plays a role in determining virulence of streptococci, inhibits phagocytosis of the organisms by leukocytes, and stimulates production of type-specific, protective antibodies by the animal host (4). These effects, together with other properties attributable to the protein, have suggested to previous investigators that the M antigen is on the exterior of group A streptococci (1, 4). Our electron microscopic findings substantiate this concept. We have found that the presence of M antigen is associated with hair-like fimbriae on the outer surfaces of several strains of group A streptococci. It appears that multiple M antigenic sites reside along the length of the fimbriae. R antigens may be associated with the same type of fimbriae. We have also obtained data suggesting the attachment of the M-associated fimbriae to the underlying portions of the cell wall. Finally, our studies suggest that M protein is not incorporated into the cell wall simultaneous to the synthesis of other portions of the wall.

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