In this task, you're given a paragraph from the research paper and your task is to generate a suitable title for the research paper based on the given paper. Under 100 words is a good title length.
Q: For many enveloped viruses, binding to a receptor(s) on a host cell acts as a first step in a series of events culminating in fusion with the host cell membrane and transfer of genetic material for replication [for review see 1,2 ]. The envelope glycoprotein (Env) trimer on the surface of HIV is responsible for receptor binding and fusion. While Env can tolerate a high degree of mutation in five variable regions (V1-V5), and also at N-linked glycosylation sites that contribute roughly half the mass of Env, the functional sites for recognition of receptor CD4 and co-receptor CXCR4/ CCR5 are conserved and essential for viral fitness. Soluble SOSIP Env trimers are structural and Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: http://www.nature.
A:
Open and Closed Structures Reveal Allostery and Pliability in the HIV-1 Envelope Spike