U12-type AT-AC introns are distinguished from the major U2-type introns by the consensus sequences of their highly conserved splicing signals. U12 introns have the 5' ss consensus sequence (G/A)TATCCTTT, the branchpoint sequence TTTCCTTAACT and the 3' ss (C/T)AG. Initial recognition of AT-AC introns involves interaction of U12 snRNP with the branch-point sequence and U11 with the 5' ss. Unlike the major splicing pathway, U11 and U12 are in a complex and interact with the pre-mRNA simultaneously, binding in an ATP-dependent manner as a di-snRNP complex and likely bridging the 5' ss and 3' ss region.
Twenty proteins have been identified in the U11/U12 di-snRNP complex including the snRNP Sm proteins B?, B, D3, D2, D1, E, F, and G which are identical to the major splicing pathway Sm proteins. A U2 snRNP core protein complex, SF3b is also found in the U11/U12 di-snRNP, including p14, a protein that interacts with the branchpoint adenosine.
SR proteins are required for formation of A complex in AT-AC splicing. The same SR proteins involved in splicing of the major introns are also active in splicing of AT-AC introns, though, as in the major pathway, there is substrate specificity.