Bifunctional enzyme that catalyzes the last two steps of purine biosynthesis. Acts as a transformylase that incorporates a formyl group to the AMP analog AICAR (5-amino-1-(5-phospho-beta-D-ribosyl)imidazole-4-carboxamide) to produce the intermediate formyl-AICAR (FAICAR). Can use both 10-formyldihydrofolate and 10-formyltetrahydrofolate as the formyl donor in this reaction. Also catalyzes the cyclization of FAICAR to IMP. Is able to convert thio-AICAR to 6-mercaptopurine ribonucleotide, an inhibitor of purine biosynthesis used in the treatment of human leukemias. Promotes insulin receptor/INSR autophosphorylation and is involved in INSR internalization.