A human type C retrovirus [
human T-cell leukemia (lymphoma) virus; HTLV], recently isolated from young adult patients with
cutaneous T-cell lymphoma or
leukemia, was not detectably related to other known animal retroviruses in molecular hybridization studies, by comparison of
reverse transcriptase and the
major core protein p24. The p24 core
protein was purified to homogeneity. The
amino acid composition, the COOH-terminal
amino acid, and the NH(2)-terminal amino acid sequence of the first 25 residues of this major internal structural
protein were determined. These results were then compared to the known structure of the internal core
protein of other retroviruses. The compositional data reveal that HTLV p24 is chemically distinct from p30-p24 of other animal retroviruses, in agreement with the earlier immunological analyses. However, HTLV p24 shares the common NH(2)-terminal
proline and COOH-terminal
leucine of all mammalian type C viral p30s. In addition, like bovine leukemia virus (BLV), HTLV lacks the common prolylleucylarginine tripeptide and the larger conserved region found near the NH(2) terminus of the other mammalian type C viral p30s. Alignment of the amino acid sequence of HTLV p24 with previously determined sequences of other
retrovirus proteins, including BLV p24, reveals statistically significant sequence homology only to BLV. The results reported here demonstrate that HTLV p24 is related to but chemically distinct from the
major core protein of other retroviruses. Similarly, previous results showed that there was no immunological crossreactivity of the p24
protein and
reverse transcriptase of HTLV with other retroviruses, including BLV, and no nucleic acid sequence homology. However, the present results, combined with the common size of the p24 and
reverse transcriptase, suggest that HTLV may be closer to BLV than any other known retrovirus.