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 (By similarity). Mammalian complex I is composed of 45 different subunits. Interacts with ETFRF1. 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. Interacts with alpha-1-microglobulin chain; this interaction is required for the maintenance of mitochondrial redox homeostasis. Belongs to the acyl carrier protein (ACP) family.