A spliceosomal complex that is formed by the cooperative binding of the heterodimeric U11/U12 snRNP to the 5' splice site and the branch point sequence. The U12-type prespliceosome includes many proteins in addition to those found in the U11/U12 heterodimeric snRNPs. Commitment to a given pair of 5' and 3' splice sites occurs at the time of prespliceosome formation.