matrix proteins
Figure 21.4 Viruses can be either complex in shape or relatively simple. This figure shows three relatively complex virions: the bacteriophage T4, with its DNA-containing head group and tail fibers that attach to host cells; adenovirus, which uses spikes from its capsid to bind to host cells; and HIV, which uses glycoproteins embedded in its envelope to bind to host cells. Notice that HIV has proteins called matrix proteins, internal to the envelope, which help stabilize virion shape. credit bacteriophage, adenovirus: modification of work by NCBI, NIH; credit HIV retrovirus: modification of work by NIAID, NIH .