Carrier of the growing fatty acid chain in fatty acid biosynthesis (By similarity). Accessory and non-catalytic subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I), which functions in the transfer of electrons from NADH to the respiratory chain (PubMed:27626371). Mammalian complex I is composed of 45 different subunits (PubMed:12611891). Interacts with ETFRF1 (PubMed:27499296). Identified in a complex composed of MALSU1, MIEF1 upstream open reading frame protein and NDUFAB1; within the trimeric complex, MIEF1 upstream open reading frame protein functions as a bridging scaffold that interacts with MALSU1 on one side, and with NDUFAB1 on the other side. The complex interacts with the mitochondrial large ribosomal subunit (PubMed:28892042, PubMed:30215512). Interacts with alpha-1-microglobulin chain; this interaction is required for the maintenance of mitochondrial redox homeostasis. In contrast to other accessory subunits of complex I, NDUFAB1 is the only subunit that is essential for cell viability in HEK293T cells. Since knockout cells lack assembled complex I and die in galactose media, this suggests that the essential role of NDUFAB1 is independent of complex I. Belongs to the acyl carrier protein (ACP) family.