Nucleotide sequence of human endogenous retrovirus genome related to the mouse mammary tumor virus genome

We determined the complete nucleotide sequence of the human endogenous retrovirus genome HERV-K10 isolated as the sequence homologous to the Syrian hamster intracisternal A-particle (type A retrovirus) genome. HERV-K10 is 9,179 base pairs long with long terminal repeats of 968 base pairs at both ends; a sequence 290 base pairs long, however, was found to be deleted. It was concluded that a composite genome having the 290-base-pair fragment is the prototype HERV-K provirus gag (666 codons), protease (334 codons), pol (937 codons), and env (618 codons) genes. The size of the protease gene product of HERV-K is essentially the same as that of A- and D-type oncoviruses but nearly twice that of other retroviruses. A comparison of the deduced amino acid sequences encoded by the pol region showed HERV-K to be closely related to types A and D retroviruses and even more so to type B retrovirus. It was noted that the env gene product of HERV-K structurally resembles the mouse mammary tumor virus (type B retrovirus) env protein, and the possible expression of the HERV-K env gene in human breast cancer cells is discussed.