G. von Heijne

Anionic phospholipids are determinants of membrane protein topology

The orientation of many membrane proteins is determined by the asymmetric distribution of positively charged amino acid residues in cytoplasmic and translocated loops. The positive‐inside rule states that loops with large amounts of these residues tend to have cytoplasmic locations. Orientations of constructs derived from the inner membrane protein leader peptidase from Escherichia coli were found to depend on the anionic phospholipid content of the membrane. Lowering the contents of anionic phospholipids facilitated membrane passage of positively charged loops. On the other hand, elevated contents of acidic phospholipids in the membrane rendered translocation more sensitive to positively charged residues. The results demonstrate that anionic lipids are determinants of membrane protein topology and suggest that interactions between negatively charged phospholipids and positively charged amino acid residues contribute to the orientation of membrane proteins.

The membrane protein universe: what's out there and why bother?

Chances are that you have come across membrane proteins many times in your professional life: ion channels, aquaporins, G‐protein‐coupled receptors, drug resistance proteins. But it is also quite likely that you have never bothered to think about what the implications are of being a membrane protein, as opposed to a soluble protein. What is special about membrane proteins in terms of structure and function, how many membrane proteins are out there, how are they made in the cell? Welcome to the membrane protein universe!

The state of the art of membrane protein structure prediction: from sequence to 3D structure

Membrane proteins constitute a very large set of yet-to-be characterized proteins mediating all the relevant life-related functions both in prokaryotes and eukaryotes. Estimates are suggesting that in whole genomes the content of this protein type may vary from 10 to 40% of the whole proteome, depending on the organism.

Evolution of Protein Sorting Signals

Sorting signals route proteins to the correct subcellular compartment and define classes of evolutionarily conserved protein motifs. Their surprisingly low degree of sequence conservation suggests that partially functional sorting signals may arise continuously during evolution, and thus that existing proteins may be continually tested in new compartments. Intertaxonic combination events such as those involved in the establishment of mitochondria and chloroplasts have necessitated the evolution of new classes of organellar import signals, and have resulted in interesting hybrids between preexisting and newly created sorting signals.

Forces on nascent polypeptides during membrane insertion and translocation via the Sec translocon

During ribosomal translation, nascent polypeptide chains (NCs) undergo a variety of physical processes that determine their fate in the cell. This study utilizes a combination of arrest peptide experiments and coarse-grained molecular dynamics to measure and elucidate the molecular origins of forces that are exerted on NCs during cotranslational membrane insertion and translocation via the Sec translocon. The approach enables deconvolution of force contributions from NC-translocon and NC-ribosome interactions, membrane partitioning, and electrostatic coupling to the membrane potential. In particular, we show that forces due to NC-lipid interactions provide a readout of conformational changes in the Sec translocon, demonstrating that lateral gate opening only occurs when a sufficiently hydrophobic segment of NC residues reaches the translocon. The combination of experiment and theory introduced here provides a detailed picture of the molecular interactions and conformational changes during ribosomal translation that govern protein biogenesis.

Glycosylation Mapping Of The Interaction Between Topogenic Sequences And The Er Translocase

Many integral membrane proteins span the hydrophobic core of a membrane with one or more a-helical segments each consisting of about 20 hydrophobic amino acids. The orientation of the hydrophobic stretch in the membrane is determined by its flanking amino acids. In general there are more positively charged residues present in cytoplasmic loops than in extra-cytoplasmic ones (von Heijne, 1986). In both prokaryotic and eukaryotic cells, proteins are targeted for secretion by N-terminal signal sequences with a common basic design: a positively charged N-terminus, a central hydrophobic stretch, and a C-terminal cleavage region that serves as a recognition site for the signal peptidase enzyme (von Heijne, 1985). Signal sequences from prokaryotes and eukaryotes look very similar and are often functionally interchangeable. They are essential for the efficient and selective targeting of the nascent protein chains either to the endoplasmic reticulum (in eukaryotes) or to the cytoplasmic membrane (in prokaryotes) (Gierasch, 1989). Signal sequences also play a central role in the interaction with the translocation machinery of the cell and in the translocation of the polypeptide chains across the membrane. Proteins are co-translationally translocated across the endoplasmic reticulum (ER) membrane. In eukaryotes two kinds of topogenic sequences are important for assembly in ER membrane: Those that initiate translocation such as signal peptides (SPs) and signalanchor sequences (SAs) and those that halt translocation, stop-transfer signals (STs). Signal peptides and signal anchor sequences differ in that SA sequences tend to have longer hydrophobic cores and lack a signal peptidase cleavage site (von Heijne, 1988).