Required for the ingression of the plasma membrane into the bud neck at the end of cytokinesis, leading to the separation of the mother and daughter cells. Stimulates the synthesis of the primary septum (PS) by CHS2. Interacts with CYK2, CYK3 and IQG1. Localizes at the contractile actinomyosin ring at the end of cytokinesis. Recruitment to the bud neck requires either CYK2 or CYK3. The C2 domain is essential for membrane ingression during cytokinesis, but not for recruitment to the actomyosin ring. Belongs to the INN1/fic1 family.