Primate tripartite motif 5alpha (TRIM5alpha) proteins mediate innate 
intracellular resistance to retroviruses. In humans, TRIM5 is located in a 
paralogous cluster that includes TRIM6, TRIM34, and TRIM22. Although TRIM6 and 
TRIM34 orthologs are found in other mammals, TRIM5 has to date been identified 
only in primates. Cow cells exhibit early blocks to infection by several 
retroviruses. We identify a cytoplasmic TRIM protein encoded by LOC505265 that 
is responsible for the restriction of infection by several lentiviruses and 
N-tropic murine leukemia virus in cow cells. Susceptibility of N-tropic murine 
leukemia virus to 505265-mediated restriction is determined primarily by residue 
110 of the viral capsid protein. Phylogenetically, cow LOC505265 segregates with 
the TRIM5/TRIM6/TRIM34 group, but is not an ortholog of known TRIM genes. The 
B30.2/SPRY domain of 505265 exhibits long variable regions, a characteristic of 
the proteins encoded by this paralogous group, and shows evidence of positive 
selection. Apparently, cows have independently evolved a retroviral restriction 
factor from the same TRIM family that spawned TRIM5 in primates. Particular 
features of this subset of cytoplasmic TRIM proteins may be conducive to the 
convergent evolution of virus-restricting factors.