RESUMEN
Proteins that fold cotranslationally may do so in a restricted configurational space, due to the volume occupied by the ribosome. How does this environment, coupled with the close proximity of the ribosome, affect the folding pathway of a protein? Previous studies have shown that the cotranslational folding process for many proteins, including small, single domains, is directly affected by the ribosome. Here, we investigate the cotranslational folding of an all-ß Ig domain, titin I27. Using an arrest peptide-based assay and structural studies by cryo-EM, we show that I27 folds in the mouth of the ribosome exit tunnel. Simulations that use a kinetic model for the force dependence of escape from arrest accurately predict the fraction of folded protein as a function of length. We used these simulations to probe the folding pathway on and off the ribosome. Our simulations-which also reproduce experiments on mutant forms of I27-show that I27 folds, while still sequestered in the mouth of the ribosome exit tunnel, by essentially the same pathway as free I27, with only subtle shifts of critical contacts from the C to the N terminus.
Asunto(s)
Conectina/química , Ribosomas/metabolismo , Conectina/genética , Conectina/metabolismo , Humanos , Cinética , Proteínas de Microfilamentos , Modelos Moleculares , Biosíntesis de Proteínas , Pliegue de Proteína , Ribosomas/química , Ribosomas/genéticaRESUMEN
This manuscript introduces a versatile system for construction of multimeric proteins to be used as substrates for atomic force microscopy. The construction makes use of a cassette system that allows modules to be cut and ligated in any combination in eight different positions. The modules can be sequenced in situ after construction. A three-module fragment can be produced that is of a size amenable to structural and biophysical analysis to check the effect of placing a protein into a multimeric construct. We show that if the parent titin modules are retained in a construct, they can act both as linkers and as an internal standard for the force measurements. Proteins that cannot be expressed solubly in an eight-module homopolymer have been expressed and subject to force measurements using this system.