Vpu is a HIV

HIV

Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...

 gene.

Vpu stands for "Viral Protein U". Vpu is involved in viral budding, enhancing virion release from the cell by counteracting BST2 (also known as tetherin

Tetherin

Tetherin also known as bone marrow stromal antigen 2 is a protein that in humans is encoded by the BST2 gene. In addition tetherin has been designated as CD317 .- Function :...

). In the absence of vpu, tetherin

Tetherin

Tetherin also known as bone marrow stromal antigen 2 is a protein that in humans is encoded by the BST2 gene. In addition tetherin has been designated as CD317 .- Function :...

 binds to the viral envelope and ties it to the cell membrane and other viral particles, impeding release of the viral particles. Additionally, Vpu targets CD4 in the ER for degradation (similar to the activity of Nef (protein)

Nef (protein)

Nef is a protein expressed by primate lentiviruses. These include human immunodeficiency viruses and simian immunodeficiency virus . Nef is one of many pathogen-expressed proteins, known as virulence factors, which function to manipulate the host's cellular machinery and thus allow infection,...

on the cell membrane). It can be found in HIV-1 but not in HIV-2.