Tài liệu Báo cáo khoa học: Integral membrane proteins in the mitochondrial outer membrane of

Integral membrane proteins in the mitochondrial outer

membrane of Saccharomyces cerevisiae

Lena Burri1

, Katherine Vascotto1,2, Ian E. Gentle1,2, Nickie C. Chan1,2, Traude Beilharz1

,

David I. Stapleton2

, Lynn Ramage3,* and Trevor Lithgow1,2

1 Department of Biochemistry and Molecular Biology, University of Melbourne, Parkville, Australia

2 Bio21 Molecular Science and Biotechnology Institute, Parkville, Australia

3 Biozentrum, University of Basel, Switzerland

Mitochondria were derived from endosymbiotic bac￾teria and the mitochondrial outer membrane shares

many features with bacterial outer membranes [1,2].

Polypeptides embedded in the outer membrane of

Gram-negative bacteria are either lipoproteins

anchored by a covalently linked lipid, or have a

b-barrel structure with eight or more antiparallel

b-strands hydrogen bonded into a cylindrical barrel

[3,4]. The b-barrel proteins have an unusual primary

structure with many strands of alternating hydrophi￾lic and hydrophobic residues and a high abundance

of aromatic residues that tend to be placed at the

start of the strands [2,5]. These b-barrel proteins

are assembled in the bacterial outer membrane in a

process mediated by the integral membrane protein

Omp85 [2,3,5–8].

Mitochondria also carry a member of the Omp85

family [8]: this protein, called Sam50, has been

shown responsible for the assembly of b-barrel pro￾teins in the mitochondrial outer membrane [9–11],

and functions together with at least two other sub￾units as part of a Sorting and Assembly Machine

(SAM) complex [7,9–11]. In addition to b-barrel pro￾teins, mitochondrial outer membranes also have pro￾teins with a-helical transmembrane domains. These

proteins appear to provide functions that were

procured after the initial endosymbiont established

itself in early eukaryotic cells, including protein

Keywords

detergent phase; mitochondria; outer

membrane; transmembrane segments

Correspondence

T. Lithgow, Department of Biochemistry and

Molecular Biology, University of Melbourne,

Parkville 3010, Australia

Fax: +61 39348 2251

Tel: +61 38344 4131

E-mail: [email protected]

*Present address

Hoffmann-La Roche Ltd., CH-4070 Basel,

Switzerland

(Received 30 October 2005, revised 29

January 2006, accepted 9 February 2006)

doi:10.1111/j.1742-4658.2006.05171.x

Mitochondria evolved from a bacterial endosymbiont ancestor in which the

integral outer membrane proteins would have been b-barrel structured

within the plane of the membrane. Initial proteomics on the outer mem￾brane from yeast mitochondria suggest that while most of the protein

components are integral in the membrane, most of these mitochondrial pro￾teins behave as if they have a-helical transmembrane domains, rather than

b-barrels. These proteins are usually predicted to have a single a-helical

transmembrane segment at either the N- or C-terminus, however, more

complex topologies are also seen. We purified the novel outer membrane

protein Om14 and show it is encoded in the gene YBR230c. Protein sequen￾cing revealed an intron is spliced from the transcript, and both transcription

from the YBR230c gene and steady-state level of the Om14 protein is dra￾matically less in cells grown on glucose than in cells grown on nonfermenta￾ble carbon sources. Hydropathy predictions together with data from limited

protease digestion show three a-helical transmembrane segments in Om14.

The a-helical outer membrane proteins provide functions derived after the

endosymbiotic event, and require the translocase in the outer mitochondrial

membrane complex for insertion into the outer membrane.

Abbreviations

DAS, dense alignment surface; PVDF, poly(vinylidene difluoride); TOM, translocase in the outer mitochondrial membrane.

FEBS Journal 273 (2006) 1507–1515 ª 2006 The Authors Journal compilation ª 2006 FEBS 1507