Self-assembly of amphiphilic dendritic dipeptides into helical pores

A conserved heterotrimeric membrane protein complex, the Sec61 or SecY complex, forms a protein-conducting channel, allowing polypeptides to be transferred across or integrated into membranes. We report the crystal structure of the complex from Methanococcus jannaschii at a resolution of 3.2 A. The structure suggests that one copy of the heterotrimer serves as a functional translocation channel. The alpha-subunit has two linked halves, transmembrane segments 1-5 and 6-10, clamped together by the gamma-subunit. A cytoplasmic funnel leading into the channel is plugged by a short helix. Plug displacement can open the channel into an 'hourglass' with a ring of hydrophobic residues at its constriction. This ring may form a seal around the translocating polypeptide, hindering the permeation of other molecules. The structure also suggests mechanisms for signal-sequence recognition and for the lateral exit of transmembrane segments of nascent membrane proteins into lipid, and indicates binding sites for partners that provide the driving force for translocation.

Sensory systems use a variety of membrane-bound receptors, including responsive ion channels, to discriminate between a multitude of stimuli. Here we describe how engineered membrane pores can be used to make rapid and sensitive biosensors with potential applications that range from the detection of biological warfare agents to pharmaceutical screening. Notably, use of the engineered pores in stochastic sensing, a single-molecule detection technology, reveals the identity of an analyte as well as its concentration.

Naturally occurring membrane channels and pores are formed from a large family of diverse proteins, peptides and organic secondary metabolites whose vital biological functions include control of ion flow, signal transduction, molecular transport and production of cellular toxins. But despite the availability of a large amount of biochemical information about these molecules, the design and synthesis of artificial systems that can mimic the biological function of natural compounds remains a formidable task. Here we present a simple strategy for the design of artificial membrane ion channels based on a self-assembled cylindrical beta-sheet peptide architecture. Our systems--essentially stacks of peptide rings--display good channel-mediated ion-transport activity with rates exceeding 10(7) ions s-1, rivalling the performance of many naturally occurring counterparts. Such molecular assemblies should find use in the design of novel cytotoxic agents, membrane transport vehicles and drug-delivery systems.