SAM50/YNL026W Summary Help

Standard Name SAM50 1
Systematic Name YNL026W
Alias OMP85 2 , TOB55 3
Feature Type ORF, Verified
Description Component of the Sorting and Assembly Machinery (SAM) complex; the SAM (or TOB) complex is located in the mitochondrial outer membrane; the complex binds precursors of beta-barrel proteins and facilitates their outer membrane insertion; homologous to bacterial Omp85 (1, 2, 3, 4 and see Summary Paragraph)
Name Description Sorting and Assembly Machinery 1
Chromosomal Location
ChrXIV:581920 to 583374 | ORF Map | GBrowse
Gene Ontology Annotations All SAM50 GO evidence and references
  View Computational GO annotations for SAM50
Molecular Function
Manually curated
Biological Process
Manually curated
Cellular Component
Manually curated
Regulators 3 genes
Classical genetics
Large-scale survey
reduction of function
30 total interaction(s) for 13 unique genes/features.
Physical Interactions
  • Affinity Capture-RNA: 1
  • Affinity Capture-Western: 16
  • Co-fractionation: 3
  • Co-purification: 4
  • Protein-peptide: 1
  • Reconstituted Complex: 1

Genetic Interactions
  • Dosage Lethality: 1
  • Dosage Rescue: 1
  • Synthetic Lethality: 2

Expression Summary
Length (a.a.) 484
Molecular Weight (Da) 54,406
Isoelectric Point (pI) 8.68
Phosphorylation PhosphoGRID | PhosphoPep Database
sequence information
ChrXIV:581920 to 583374 | ORF Map | GBrowse
Last Update Coordinates: 2011-02-03 | Sequence: 1997-01-28
Subfeature details
Most Recent Updates
Coordinates Sequence
CDS 1..1455 581920..583374 2011-02-03 1997-01-28
Retrieve sequences
Analyze Sequence
S288C only
S288C vs. other species
S288C vs. other strains
External Links All Associated Seq | Entrez Gene | Entrez RefSeq Protein | MIPS | Search all NCBI (Entrez) | UniProtKB
Primary SGDIDS000004971

About mitochondrial import

While the mitochondrial genome encodes a handful of proteins, most of the hundreds of proteins that reside in the mitochondrion are encoded by nuclear genes, translated in the cytoplasm, and imported into mitochondria via a series of complex molecular machines (see 5, 6 for review). Many of the proteins imported into mitochondria are involved in respiration, which is not an essential process: S. cerevisiae is able to carry out either fermentative growth on carbon sources such as glucose, or respiratory growth on nonfermentable carbon sources such as glycerol and ethanol. However, since maintenance of the mitochondrial compartment is essential to life, mutations that completely disrupt mitochondrial import are lethal.

about the SAM complex

The sorting and assembly machinery (SAM) complex, also known as the translocase of outer membrane beta-barrel proteins (TOB), is required for the correct insertion of beta-barrel proteins into the mitochondrial outer membrane (7). The core of this complex, which is located in the outer membrane, is composed of Sam50p/Tob55p, itself a beta-barrel protein; Sam37p/Mas37p; and Sam35p/Tob38p (1, 8, 9, 10, 11). Mdm10p, a protein first discovered for its role in mitochondrial morphology, also associates with the SAM complex (12).

Beta-barrel proteins are first translocated across the outer membrane by the translocase of the outer mitochondrial membrane (TOM) complex. After transit through the TOM complex into the intermembrane space, both of the complexes of small TIM proteins that reside there (Tim8p-Tim13p complex and Tim9p-Tim10p) are involved in delivery of the beta-barrel proteins to the SAM complex (13). The SAM complex then mediates insertion of the proteins into the mitochondrial outer membrane (14). The final steps of the process require Mdm10p as well as two other proteins implicated in maintenance of mitochondrial morphology, Mdm12p and Mmm1p, which themselves form a complex with Mdm10p (12). In addition to Sam50p/Tob55p, the beta-barrel proteins imported by this route include porin (Por1p or VDAC), the most abundant outer membrane protein; Mdm10p; and Tom40p, which comprises the pore of the TOM complex. All of these substrate proteins have a SAM complex recognition motif termed the beta-signal (14). The SAM complex is also required for correct insertion of some other subunits of the TOM complex, which do not have a beta-barrel structure, into the outer membrane: the entire complex is required for assembly of Tom22p into the TOM complex, while Sam37p only is required for assembly of Tom5p, Tom6p, and Tom7p (15).

about SAM50

Sam50p is an essential constituent of the SAM complex and is conserved across prokaryotes and eukaryotes (1, 3). Conditional sam50 mutations selectively affect the import of beta-barrel proteins (1, 3, 16). Sam50p has a beta-barrel structure and also contains an N-terminal polypeptide transport-associated (POTRA) domain that is exposed to the intermembrane space. There is conflicting evidence on whether the N-terminal POTRA domain of Sam50p is involved in recognition of beta-barrel precursor proteins by the SAM complex, or whether Sam35p fills this role (17, 14). The role of Sam50p in the later steps of import is clear: the isolated protein can integrate into a lipid bilayer and form a functional channel in vitro, and is thought to act in vivo by allowing beta-barrel proteins to fold within its channel and then releasing them laterally into the outer membrane (3, 14).

Last updated: 2009-03-17 Contact SGD

References cited on this page View Complete Literature Guide for SAM50
1) Kozjak V, et al.  (2003) An essential role of Sam50 in the protein sorting and assembly machinery of the mitochondrial outer membrane. J Biol Chem 278(49):48520-3
2) Gentle I, et al.  (2004) The Omp85 family of proteins is essential for outer membrane biogenesis in mitochondria and bacteria. J Cell Biol 164(1):19-24
3) Paschen SA, et al.  (2003) Evolutionary conservation of biogenesis of beta-barrel membrane proteins. Nature 426(6968):862-6
4) Habib SJ, et al.  (2005) Assembly of the TOB complex of mitochondria. J Biol Chem 280(8):6434-40
5) Neupert W and Herrmann JM  (2007) Translocation of proteins into mitochondria. Annu Rev Biochem 76:723-49
6) Mokranjac D and Neupert W  (2009) Thirty years of protein translocation into mitochondria: unexpectedly complex and still puzzling. Biochim Biophys Acta 1793(1):33-41
7) Paschen SA, et al.  (2005) Biogenesis of beta-barrel membrane proteins of mitochondria. Trends Biochem Sci 30(10):575-82
8) Wiedemann N, et al.  (2003) Machinery for protein sorting and assembly in the mitochondrial outer membrane. Nature 424(6948):565-71
9) Milenkovic D, et al.  (2004) Sam35 of the mitochondrial protein sorting and assembly machinery is a peripheral outer membrane protein essential for cell viability. J Biol Chem 279(21):22781-5
10) Waizenegger T, et al.  (2004) Tob38, a novel essential component in the biogenesis of beta-barrel proteins of mitochondria. EMBO Rep 5(7):704-9
11) Ishikawa D, et al.  (2004) Two novel proteins in the mitochondrial outer membrane mediate beta-barrel protein assembly. J Cell Biol 166(5):621-7
12) Meisinger C, et al.  (2007) The morphology proteins Mdm12/Mmm1 function in the major beta-barrel assembly pathway of mitochondria. EMBO J 26(9):2229-39
13) Wiedemann N, et al.  (2004) Biogenesis of the protein import channel Tom40 of the mitochondrial outer membrane: intermembrane space components are involved in an early stage of the assembly pathway. J Biol Chem 279(18):18188-94
14) Kutik S, et al.  (2008) Dissecting membrane insertion of mitochondrial beta-barrel proteins. Cell 132(6):1011-24
15) Stojanovski D, et al.  (2007) Alternative function for the mitochondrial SAM complex in biogenesis of alpha-helical TOM proteins. J Cell Biol 179(5):881-93
16) Habib SJ, et al.  (2007) The N-terminal domain of Tob55 has a receptor-like function in the biogenesis of mitochondrial beta-barrel proteins. J Cell Biol 176(1):77-88
17) Kotaka A, et al.  (2009) Efficient generation of recessive traits in diploid sake yeast by targeted gene disruption and loss of heterozygosity. Appl Microbiol Biotechnol 82(2):387-95