Two types of HTLV-1 particles are released from MT-2 cells.

The MT-2 cell line transformed by human T-cell leukemia virus type 1 (HTLV-1) contains one complete provirus and seven defective proviruses. Four defective genomes have an identical structure (LTR-MA-deltaCA-pX-LTR) with an open reading frame that spans from MA to pX, giving rise to a 3.4-kb (24S) RNA transcript encoding a chimeric Gag-pX protein, p28. MT-2 cells release two distinct types of virions. The major "classic" type of particle has a buoyant density of 1.155-1.16 g/cm3 and contains the standard HTLV-I structural proteins and reverse transcriptase (RT). In addition, about 5% of particles are "light," approximately 1.12 g/cm3, and contain p28, RT activity, and the 3.4-kb RNA transcript. RT-PCR and in vitro translation indicate that some of the classic HTLV-1 particles package 3.4-kb RNA as well as full-length 8.5-kb RNA. In addition to matrix features, the p28 protein has a motif resembling a zinc finger at the C-terminal, pX0 region, which may play a role in the assembly of the defective light virions.