Chemokine:

Chemokines are proteins some of which are membrane bound as seen in HIV infection, and are critical to infection process of HIV into cells.  The following link demonstrates this process:

http://www.youtube.com/watch?v=oneYI0fhGa0

http://www.sabiosciences.com/iapp/chemokine.html

http://www.sciencemag.org/content/274/5295/1988.1.full

http://pathmicro.med.sc.edu/lecture/hiv7.htm

http://jpet.aspetjournals.org/content/297/1/1.full

http://thurj.org/research/2011/02/1238/

http://student.biology.arizona.edu/honors97/group12/whatis1.htm