Catalytic subunit of the PI3K complex that mediates formation of phosphatidylinositol 3-phosphate which plays a key role in initiation and maturation of autophagosomes. Involved in the transport of lysosomal enzyme precursors to lysosomes. Required for the abcission step in cytokinesis. Required for transport from early to late endosomes. Heterodimer. This subunit, part of a complex composed of regulatory and catalytic subunits, associates with regulatory subunit PIK3R4. Forms a complex with BECN1, PIK3R4 and either UVRAG and KIAA0226/Rubicon, or with ATG14. In this complex, presence of UVRAG and ATG14 are mutually exclusive. Part of a complex composed of PIK3R4 and PIK3CB. Interacts with RAB7A in the presence of PIK3R4. Ubiquitously expressed, with a highest expression in skeletal muscle. Belongs to the PI3/PI4-kinase family. Note: This description may include information from UniProtKB.