A Conceptual Mathematical Model of the Dynamic Self-Organisation of Distinct Cellular Organelles

Formation, degradation and renewal of cellular organelles is a dynamic process based on permanent budding, fusion and inter-organelle traffic of vesicles. These processes include many regulatory proteins such as SNAREs, Rabs and coats. Given this complex machinery, a controversially debated issue is the definition of a minimal set of generic mechanisms necessary to enable the self-organization of organelles differing in number, size and chemical composition. We present a conceptual mathematical model of dynamic organelle formation based on interacting vesicles which carry different types of fusogenic proteins (FP) playing the role of characteristic marker proteins. Our simulations (ODEs) show that a de novo formation of non-identical organelles, each accumulating a different type of FP, requires a certain degree of disproportionation of FPs during budding. More importantly however, the fusion kinetics must indispensably exhibit positive cooperativity among these FPs, particularly for the formation of larger organelles. We compared different types of cooperativity: sequential alignment of corresponding FPs on opposite vesicle/organelles during fusion and pre-formation of FP-aggregates (equivalent, e.g., to SNARE clusters) prior to fusion described by Hill kinetics. This showed that the average organelle size in the system is much more sensitive to the disproportionation strength of FPs during budding if the vesicular transport system gets along with a fusion mechanism based on sequential alignments of FPs. Therefore, pre-formation of FP aggregates within the membranes prior to fusion introduce robustness with respect to organelle size. Our findings provide a plausible explanation for the evolution of a relatively large number of molecules to confer specificity on the fusion machinery compared to the relatively small number involved in the budding process. Moreover, we could speculate that a specific cooperativity which may be described by Hill kinetics (aggregates or Rab/SNARE complex formation) is suitable if maturation/identity switching of organelles play a role (bistability).     

An essential role of Sam50 in the protein sorting and assembly machinery of the mitochondrial outer membrane

The preprotein translocase of the outer mitochondrial membrane (TOM complex) contains one essential subunit, the channel Tom40. The assembly pathway of the precursor of Tom40 involves the TOM complex and the sorting and assembly machinery (SAM complex) with the non-essential subunit Mas37. We have identified Sam50, the second essential protein of the mitochondrial outer membrane. Sam50 contains a beta-barrel domain conserved from bacteria to man and is a subunit of the SAM complex. Yeast mutants of Sam50 are defective in the assembly pathways of Tom40 and the abundant outer membrane protein porin, while the import of matrix proteins is not affected. Thus the protein sorting and assembly machinery of the mitochondrial outer membrane involves an essential, conserved protein.     

Germacra-12,6β-olides from Montanoa revealii and M. mollissima

Montanoa revealii and M. mollissima yielded a series of germacra-12,6β-olides similar to those previously reported from M. hibiscifolia and M. pteropoda.     

The Inversion Effect in Biological Motion Perception: Evidence for a “Life Detector”?

If biological-motion point-light displays are presented upside down, adequate perception is strongly impaired. Reminiscent of the inversion effect in face recognition, it has been suggested that the inversion effect in biological motion is due to impaired configural processing in a highly trained expert system. Here, we present data that are incompatible with this view. We show that observers can readily retrieve information about direction from scrambled point-light displays of humans and animals. Even though all configural information is entirely disrupted, perception of these displays is still subject to a significant inversion effect. Inverting only parts of the display reveals that the information about direction, as well as the associated inversion effect, is entirely carried by the local motion of the feet. We interpret our findings in terms of a visual filter that is tuned to the characteristic motion of the limbs of an animal in locomotion and hypothesize that this mechanism serves as a general detection system for the presence of articulated terrestrial animals.