Bifunctional enzyme that catalyzes the last two steps of purine biosynthesis (PubMed:29072452). 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) (PubMed:29072452). Also displays cyclohydrolase activity involving the cyclization of FAICAR to IMP. Can use both 10-formyldihydrofolate and 10-formyltetrahydrofolate as the formyl donor in this reaction. Also catalyzes the cyclization of FAICAR to IMP. Promotes insulin receptor/INSR autophosphorylation and is involved in INSR internalization (By similarity). (6S)-10-formyltetrahydrofolate + 5-amino-1-(5-phospho-beta-D-ribosyl)imidazole-4-carboxamide = (6S)-5,6,7,8-tetrahydrofolate + 5-formamido-1-(5-phospho-D-ribosyl)imidazole-4-carboxamide 10-formyldihydrofolate + 5-amino-1-(5-phospho-beta-D-ribosyl)imidazole-4-carboxamide = 5-formamido-1-(5-phospho-D-ribosyl)imidazole-4-carboxamide + 7,8-dihydrofolate H2O + IMP = 5-formamido-1-(5-phospho-D-ribosyl)imidazole-4-carboxamide AMP and XMP inhibit AICAR formyltransferase activity (By similarity). AICAR formyltransferase activity is inhibited by N-(6-fluoro-1-oxo-1,2-dihydroisoquinolin-7-yl)-5- [(3R)-3-hydroxypyrrolidin-1-yl]thiophene-2-sulfonamide (LSN 3213128), which acts as a tumor suppression in cancer cell lines (PubMed:29072452). Purine metabolism; IMP biosynthesis via de novo pathway; 5-formamido-1-(5-phospho-D-ribosyl)imidazole-4-carboxamide from 5-amino-1-(5-phospho-D-ribosyl)imidazole-4-carboxamide (10-formyl THF route): step 1/1. Purine metabolism; IMP biosynthesis via de novo pathway; IMP from 5-formamido-1-(5-phospho-D-ribosyl)imidazole-4-carboxamide: step 1/1. Homodimer. Associates with internalized INSR complexes on Golgi/endosomal membranes. Interacts with INSR; ATIC together with PRKAA2/AMPK2 and HACD3/PTPLAD1 is proposed to be part of a signaling network regulating INSR autophosphorylation and endocytosis. The IMP cyclohydrolase activity resides in the N-terminal region. The de novo purine synthesis pathway includes 10 sequential steps, beginning with phosphoribosyl pyrophosphate and ending with inositol monophosphate (IMP), the first purin compound of the pathway. Belongs to the PurH family.