MLN64 contains a domain with homology to the steroidogenic acute regulatory protein (StAR) that stimulates steroidogenesis

Department of Medicine, University of California, San Diego, San Diego, California, United States
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 09/1997; 94(16). DOI: 10.1073/pnas.94.16.8462
Source: PubMed Central


MLN64 is a protein that is highly expressed in certain breast carcinomas. The C terminus of MLN64 shares significant homology with the steroidogenic acute regulatory protein (StAR), which plays a key role in steroid hormone biosynthesis by enhancing the intramitochondrial translocation of cholesterol to the cholesterol side-chain cleavage enzyme. We tested the ability of MLN64 to stimulate steroidogenesis by using COS-1 cells cotransfected with plasmids expressing the human cholesterol side-chain cleavage enzyme system and wild-type and mutant MLN64 proteins. Wild-type MLN64 increased pregnenolone secretion in this system 2-fold. The steroidogenic activity of MLN64 was found to reside in the C terminus of the protein, because constructs from which the C-terminal StAR homology domain was deleted had no steroidogenic activity. In contrast, removal of N-terminal sequences increased MLN64’s steroidogenesis-enhancing activity. MLN64 mRNA was found in many human tissues, including the placenta and brain, which synthesize steroid hormones but do not express StAR. Western blot analysis revealed the presence of lower molecular weight immunoreactive MLN64 species that contain the C-terminal sequences in human tissues. Homologs of both MLN64 and StAR were identified in Caenorhabditis elegans, indicating that the two proteins are ancient. Mutations that inactivate StAR were correlated with amino acid residues that are identical or similar among StAR and MLN64, indicating that conserved motifs are important for steroidogenic activity. We conclude that MLN64 stimulates steroidogenesis by virtue of its homology to StAR.

Download full-text


Available from: Michael E Baker
  • Source
    • "Normal human skin tissue also expresses MLN64 (metastatic lymph node 64), a functional homolog of StAR (Slominski et al., 2004). MLN64 shares sequence similarities with StAR and is suggested to regulate cholesterol transport in cells which do not express StAR (Watari et al., 1997). These molecules were found to be down-regulated in psoriasis and atopic dermatitis, suggesting that steroidogenesis is impaired under those pathologic conditions, which are commonly treated with GC derivatives (Hannen et al., 2011). "
    [Show abstract] [Hide abstract]
    ABSTRACT: Systemic glucocorticoids (GCs) mainly originate from de novo synthesis in the adrenal cortex under the control of the hypothalamus-pituitary-adrenal (HPA)-axis. However, research during the last 1-2 decades has revealed that additional organs express the necessary enzymes and have the capacity for de novo synthesis of biologically active GCs. This includes the thymus, intestine, skin and the brain. Recent research has also revealed that locally synthesized GCs most likely act in a paracrine or autocrine manner and have significant physiological roles in local homeostasis, cell development and immune cell activation. In this review, we summarize the nature, regulation and known physiological roles of extra-adrenal GC synthesis. We specifically focus on the thymus in which GC production (by both developing thymocytes and stromal cells) has an important role in the maintenance of proper immunological function.
    Full-text · Article · May 2013 · Molecular and Cellular Endocrinology
  • Source
    • "A role for MLN64 in cholesterol trafficking was first proposed based upon its sequence homology to the functional region of the steroidogenic acute regulatory (StAR) protein [2], [6]. X-ray crystallographic analysis of the START domain revealed the existence of a hydrophobic cavity that could potentially accommodate a single cholesterol molecule [7], supporting the idea that MLN64 could function as a cholesterol transporter. "
    [Show abstract] [Hide abstract]
    ABSTRACT: MLN64 is an integral membrane protein localized to the late endosome and plasma membrane that is thought to function as a mediator of cholesterol transport from endosomal membranes to the plasma membrane and/or mitochondria. The protein consists of two distinct domains: an N-terminal membrane-spanning domain that shares homology with the MENTHO protein and a C-terminal steroidogenic acute regulatory protein (StAR)-related lipid transfer (START) domain that binds cholesterol. To further characterize the MLN64 protein, full-length and truncated proteins were overexpressed in cells and the effects on MLN64 trafficking and endosomal morphology were observed. To gain insight into MLN64 function, affinity chromatography and mass spectrometric techniques were used to identify potential MLN64 interacting partners. Of the 15 candidate proteins identified, 14-3-3 was chosen for further characterization. We show that MLN64 interacts with 14-3-3 in vitro as well as in vivo and that the strength of the interaction is dependent on the 14-3-3 isoform. Furthermore, blocking the interaction through the use of a 14-3-3 antagonist or MLN64 mutagenesis delays the trafficking of MLN64 to the late endosome and also results in the dispersal of endocytic vesicles to the cell periphery. Taken together, these studies have determined that MLN64 is a novel 14-3-3 binding protein and indicate that 14-3-3 plays a role in the endosomal trafficking of MLN64. Furthermore, these studies suggest that 14-3-3 may be the link by which MLN64 exerts its effects on the actin-mediated endosome dynamics.
    Full-text · Article · Apr 2012 · PLoS ONE
  • Source
    • "1B) is also a complex biochemical pathway that involves numerous cytochrome P450 (Cyp) and hydroxysteroid dehydrogenases (Hsd) enzymes (Payne and Hales 2004). In vertebrates, steroidogenesis is initiated by Cyp11a enzymes that produce pregnenolone from cholesterol transferred from the outer to the inner membrane of mitochondria by Star (for steroidogenic acute regulatory protein, also known as Stard1) or by Stard3 (for Star-related lipid transfer domain containing 3, also known as Mln64) proteins (Clark et al. 1994; Watari et al. 1997; Stocco 2001). Pregnenolone is transformed into progesterone by 3b-hydroxysteroid dehydrogenase/D 5 -D 4 isomerase (Hsd3b) or to dehydroepiandrosterone (DHEA) by Cyp17. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Although the physiological relevance of retinoids and steroids in vertebrates is very well established, the origin and evolution of the genetic machineries implicated in their metabolic pathways is still very poorly understood. We investigated the evolution of these genetic networks by conducting an exhaustive survey of components of the retinoid and steroid pathways in the genome of the invertebrate chordate amphioxus (Branchiostoma floridae). Due to its phylogenetic position at the base of chordates, amphioxus is a very useful model to identify and study chordate versus vertebrate innovations, both on a morphological and a genomic level. We have characterized more than 220 amphioxus genes evolutionarily related to vertebrate components of the retinoid and steroid pathways and found that, globally, amphioxus has orthologs of most of the vertebrate components of these two pathways, with some very important exceptions. For example, we failed to identify a vertebrate-like machinery for retinoid storage, transport, and delivery in amphioxus and were also unable to characterize components of the adrenal steroid pathway in this invertebrate chordate. The absence of these genes from the amphioxus genome suggests that both an elaboration and a refinement of the retinoid and steroid pathways took place at the base of the vertebrate lineage. In stark contrast, we also identified massive amplifications in some amphioxus gene families, most extensively in the short-chain dehydrogenase/reductase superfamily, which, based on phylogenetic and genomic linkage analyses, were likely the result of duplications specific to the amphioxus lineage. In sum, this detailed characterization of genes implicated in retinoid and steroid signaling in amphioxus allows us not only to reconstruct an outline of these pathways in the ancestral chordate but also to discuss functional innovations in retinoid homeostasis and steroid-dependent regulation in both cephalochordate and vertebrate evolution.
    Full-text · Article · Aug 2011 · Genome Biology and Evolution
Show more