What does U2AF65 bind during the splicing process?

U2AF65 is a critical component of the spliceosome, which is responsible for the removal of introns from pre-mRNA transcripts during the splicing process. Specifically, U2AF65 binds to the pre-mRNA's pyrimidine-rich tract, also known as the Py tract, that is located upstream of the 3' splice site. The Py tract is essential for the recognition and assembly of the spliceosome on the mRNA.

By binding to the Py tract, U2AF65 helps recruit U2 snRNP to the branch point, a step that is crucial for the correct splicing of the mRNA. This binding is integral to facilitating the assembly of other splicing factors and ultimately ensuring that splicing proceeds with fidelity.