Vpr (96 amino acids)

Once the virus has fused with the cell membrane, it loses its coat in the cytoplasm and the nucleoprotein complexes of RNA and protein are transported to the nucleus. Partial transcription of the RNA occurs before arrival at the nucleus. HIV can infect non-dividing cells such as macrophages and Vpr serves as a nuclear localization signal. This occurs without the breakdown of the nuclear membrane that occurs at mitosis. There are two alpha helices at the N terminal of Vpr that are important for this since there is not a canonical nuclear localization signal. The Vpr signal targets the nucleoprotein complexes to the nuclear pores. The protein is incorporated into virions and interacts with p6 and the C-terminus is MA.

Vpr also induces cell cycle arrest at G2 which allows the virus to replicate more efficiently. This appears to occur by the formation of holes in the membrane. This allows proteins that were formerly exclusively nuclear or cytoplasmic to mix together. Among the proteins that move from their original site because of these holes are Cdc25 (usually cytoplasmic) and Wee1 (usually in the nucleus). Both Cdc25 and Wee1 regulate the phosphorylation of Cdc2  (p34cdc2 cyclin dependent kinase) which is a protein involved in the control of the cell cycle. Wee1 keeps Cdc2 phosphorylated because it is nuclear and the phosphoryated form of Cdc2 is inactive. When Cdc2 moves to the cytoplasm, it is activated by loss of the phosphate group (by cytoplasmic Cdc25) and this may block the cell cycle at G2. This does not happen with Vpr mutants impaired for G2 arrest.  It is also possible that the holes cause by Vpr in the nuclear lamina may alter cell division. 

Vpr influences mutation rates during DNA synthesis and may form an ion channel

Further information: Dynamic Disruptions in Nuclear Envelope Architecture and Integrity Induced by HIV-1 Vpr  by Carlos M. C. de Noronha et al

For further information see: Alan Frankel and John A. T. Young, HIV-1: Fifteen proteins and an RNA Annual Review of Biochemistry 67: 1-25, 1998