Is formation of visible channels in a phospholipid bilayer by botulinum neurotoxin type B sensitive to its disulfide?

Botulinum neurotoxin, produced by Clostridium botulinum as a approximately 150-kDa single-chain protein, is nicked proteolytically either endogenously or exogenously. The approximately 50- and approximately 100-kDa chains of the dichain molecule remain held together by an interchain disulfide bridge and noncovalent interactions. The neurotoxin binds to receptors of the target cell and is internalized by endocytosis.

Association of Nef with the human immunodeficiency virus type 1 core.

Highly conserved among primate lentiviruses, the human immunodeficiency virus type 1 (HIV-1) Nef protein enhances viral infectivity by an unknown mechanism. Nef-defective virions are blocked at a stage of the HIV-1 life cycle between entry and reverse transcription, possibly virus uncoating. Nef is present in purified HIV-1 particles; however, it has not been determined whether Nef is specifically recruited into HIV-1 particles or whether virion-associated Nef plays a functional role in HIV-1 replication.

Ultrastructural morphology of the hnRNP C protein tetramer.

The C protein tetramer is one of three heterotetramers which comprise the majority of the protein mass of mammalian 40S nuclear ribonucleoprotein particles (hnRNP particles). The events of RNA processing occur while the nascent transcripts are packaged in these structures. The C protein tetramer contains three monomers of C1 and one C2 monomer [i.

An ultrastructural characterization of in vitro-assembled hnRNP C protein-RNA complexes.

In mammalian cells approximately 700 nucleotide lengths of pre-mRNA are packaged during transcription by a unique group of abundant nuclear proteins to form a repeating array of regular ribonucleoprotein complexes termed 30-40S heterogeneous nuclear ribonucleoprotein particles (hnRNP particles). We have used electron microscopy to examine complexes that form when in vitro-transcribed RNA is bound by one of the purified native core-particle proteins which comprise the 40S monoparticle (the C protein tetramer). Negatively stained images of the C protein tetramer bound to particle-length RNA (700 nt) demonstrate that three tetramers bind each RNA molecule to form a stable closed triangular complex.

The C-protein tetramer binds 230 to 240 nucleotides of pre-mRNA and nucleates the assembly of 40S heterogeneous nuclear ribonucleoprotein particles.

A series of in vitro protein-RNA binding studies using purified native (C1)3C2 and (A2)3B1 tetramers, total soluble heterogeneous nuclear ribonucleoprotein (hnRNP), and pre-mRNA molecules differing in length and sequence have revealed that a single C-protein tetramer has an RNA site size of 230 to 240 nucleotides (nt). Two tetramers bind twice this RNA length, and three tetramers fold monoparticle lengths of RNA (700 nt) into a unique 19S triangular complex. In the absence of this unique structure, the basic A- and B-group proteins bind RNA to form several different artifactual structures which are not present in preparations of native hnRNP and which do not function in hnRNP assembly.