Membrane proteins are of outstanding importance in biology, drug discovery and vaccination. A common limiting factor in research and applications involving membrane proteins is the ability to solubilize and stabilize membrane proteins. Although detergents represent the major means for solubilizing membrane proteins, they are often associated with protein instability and poor applicability in structural and biophysical studies. Here, we present a novel lipoprotein nanoparticle system that allows for the reconstitution of membrane proteins into a lipid environment that is stabilized by a scaffold of Saposin proteins. We showcase the applicability of the method on two purified membrane protein complexes as well as the direct solubilization and nanoparticle-incorporation of a viral membrane protein complex from the virus membrane. We also demonstrate that this lipid nanoparticle methodology facilitates high-resolution structural studies of membrane proteins in a lipid environment by single-particle electron cryo-microscopy (cryo-EM) and allows for the stabilization of the HIV-envelope glycoprotein in a functional state.

SummaryProton-dependent oligopeptide transporters (POTs) are important for uptake of dietary di- and tripeptides in many organisms, and in humans are also involved in drug absorption. These transporters accept a wide range of substrates, but the structural basis for how different peptide side chains are accommodated has so far remained obscure. Twenty-eight peptides were screened for binding to PepTSt from Streptococcus thermophilus, and structures were determined of PepTSt in complex with four physicochemically diverse dipeptides, which bind with millimolar affinity: Ala-Leu, Phe-Ala, Ala-Gln, and Asp-Glu. The structures show that PepTSt can adapt to different peptide side chains through movement of binding site residues and water molecules, and that a good fit can be further aided by adjustment of the position of the peptide itself. Finally, structures were also determined in complex with adventitiously bound HEPES, polyethylene glycol, and phosphate molecules, which further underline the adaptability of the binding site.

Bacteriophage T5 represents a large family of lytic Siphoviridae infecting Gram-negative bacteria. The low-resolution structure of T5 showed the T‫31؍‬ geometry of the capsid and the unusual trimeric organization of the tail tube, and the assembly pathway of the capsid was established. Although major structural proteins of T5 have been identified in these studies, most of the genes encoding the morphogenesis proteins remained to be identified. Here, we combine a proteomic analysis of T5 particles with a bioinformatic study and electron microscopic immunolocalization to assign function to the genes encoding the structural proteins, the packaging proteins, and other nonstructural components required for T5 assembly. A head maturation protease that likely accounts for the cleavage of the different capsid proteins is identified. Two other proteins involved in capsid maturation add originality to the T5 capsid assembly mechanism: the single head-to-tail joining protein, which closes the T5 capsid after DNA packaging, and the nicking endonuclease responsible for the single-strand interruptions in the T5 genome. We localize most of the tail proteins that were hitherto uncharacterized and provide a detailed description of the tail tip composition. Our findings highlight novel variations of viral assembly strategies and of virion particle architecture. They further recommend T5 for exploring phage structure and assembly and for deciphering conformational rearrangements that accompany DNA transfer from the capsid to the host cytoplasm. Bacteriophage T5 is a member of the Siphoviridae family infecting Escherichia coli. It consists of an icosahedral capsid containing a large molecule of double-stranded DNA (dsDNA) (121.75 kbp) attached to a long flexible noncontractile tail. The complete genomes of two wild-type T5 strains (GenBank accession numbers AY587007 [1] and AY543070) and of the heat-stable deletion mutant T5st0 (GenBank accession number AY692264 [this study]) have been sequenced. Moreover, the genomes of T5-related phages H8 (2), EPS7 (3), SPC35 (4), AKVF3 (5), pVp-1 (6), and My1 (7) exhibit high sequence similarity to T5. Of the 159 to 174 genes predicted in each genome, about 70 were assigned functions on the basis of similarity searches and/or previous genetic studies. Most of the identified genes are related to nucleotide metabolism, DNA replication, recombination, and various enzymatic functions. Despite the fact that the major structural proteins of T5 have been identified (8), the functions of 13 of the 23 late genes encoding the structural and morphogenesis proteins remain to be ascertained.The overall structure of T5 was solved by cryo-electron microscopy (cryo-EM) and image reconstruction (9). The large icosahedral capsid consists of the coat protein pb8, arranged as 11 pentamers at the vertices and 120 hexamers on the faces. The 12th vertex is occupied by a dodecamer of the portal protein pb7. The early events of T5 capsid assembly have been partly deciphered (10). The initial shell (prohead I) is assemb...