Primary structure analysis of the major internal protein p24 of human type C T-cell leukemia virus

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.