Combination of hydrophobicity and codon usage bias determines sorting of model K+ channel protein to either mitochondria or endoplasmic reticulum.

When the K+ channel-like protein Kesv from Ectocarpus siliculosus virus 1 is heterologously expressed in mammalian cells, it is sorted to the mitochondria. This targeting can be redirected to the endoplasmic reticulum (ER) by altering the codon usage in distinct regions of the gene or by inserting a triplet of hydrophobic amino acids (AAs) into the protein's C-terminal transmembrane domain (ct-TMD). Systematic variations in the flavor of the inserted AAs and/or its codon usage show that a positive charge in the inserted AA triplet alone serves as strong signal for mitochondria sorting. In cases of neutral AA triplets, mitochondria sorting are favored by a combination of hydrophilic AAs and rarely used codons; sorting to the ER exhibits the inverse dependency. This propensity for ER sorting is particularly high when a common codon follows a rarer one in the AA triplet; mitochondria sorting in contrast is supported by codon uniformity. Since parameters like positive charge, hydrophobic AAs, and common codons are known to facilitate elongation of nascent proteins in the ribosome the data suggest a mechanism in which local changes in elongation velocity and co-translational folding in the ct-TMD influence intracellular protein sorting.

Investigating Cellulose Binding of Peptides Derived from Carbohydrate Binding Module 1.

Carbohydrate-binding modules (CBM) have emerged as useful tools for a wide range of tasks, including the use as purification tags or for cellulose fiber modification. For this purpose, the CBM needs to be attached to a target protein leading to large constructs. We investigated if short peptides from the carbohydrate binding site of CBMs can bind in a similar way as native, full-length CBMs to nanocrystalline cellulose (NCC) or cotton linter paper. We designed our cellulose-binding peptides to be less hydrophobic and shorter than those previously reported. Starting from the binding site of Cel7A-CBM1, we incorporated the essential amino acids involved in cellulose binding into our peptides. These peptides, as well as control peptides with scrambled sequences or a lack of essential amino acids, bound to cellulose with similar affinity as CBM regardless of their secondary structure, sequence, or hydrophobicity. This unspecific mode of cellulose binding displayed by the presented peptides may be exploited to functionalize cellulose-based biomaterials by means of peptide-conjugates.