Key component of the cytosolic iron-sulfur protein assembly (CIA) complex, a multiprotein complex that mediates the incorporation of iron-sulfur cluster into apoproteins specifically involved in DNA metabolism and genomic integrity (By similarity). In the CIA complex, MMS19 acts as an adapter between early-acting CIA components and a subset of cellular target iron-sulfur proteins (By similarity). Essential for diploid cell cycles, organ growth and development (PubMed:29361561). Regulates mitosis by binding to Xpd and thereby competing with the Xpd-mediated repression on the Cdk-activating kinase (CAK) complex (PubMed:29361561). Regulates the centrosomal localization of the MT regulator tacc, a downstream target of aurA kinase (PubMed:33211700). Binds to microtubules (MT) (PubMed:33211700). Regulates spindle and astral MT growth, MT stability and bundling (PubMed:33211700). In neuroblasts, necessary for timely and coordinated spindle assembly and orientation which is necessary for mitotic progression (PubMed:33211700). In young embryos, the maternal protein is important for progression through mitosis (PubMed:29361561). Component of the CIA complex (By similarity). Interacts with Xpd and galla-2 (PubMed:29361561). Binds to microtubules (PubMed:33211700). Localizes to the nucleus at the beginning of metaphase. Expressed in embryos (at protein level). Shows slowed larval development with lack of imaginal disks and microcephaly with deformed and underdeveloped optical lobes (PubMed:29361561, PubMed:33211700). Death occurs at the third larval instar stage (PubMed:29361561, PubMed:33211700). Reduced levels of Xpd in embryos (PubMed:29361561). Results in higher proportion of neuroblasts in mitosis with shorter spindle and less dense astral microtubules with defective assembly and orientation (PubMed:33211700). Loss of maternal Mms19 causes cell cycle defects in young embryos (PubMed:29361561). Belongs to the MET18/MMS19 family.