Irreducible complexity: mitotic spindle orientation (Introduction)

by David Turell , Saturday, January 09, 2016, 15:37 (3242 days ago) @ David Turell

In cell division the mitotic spindle must have proper orientation so the cells line up properly to create sheets of functional cells:-http://elifesciences.org/content/5/e10147#F1-"A molecular complex scaffolded by the GK protein-interaction domain (GKPID) mediates spindle orientation in diverse animal taxa by linking microtubule motor proteins to a marker protein on the cell cortex localized by external cues. Here we illuminate how this complex evolved and commandeered control of spindle orientation from a more ancient mechanism. The complex was assembled through a series of molecular exploitation events, one of which - the evolution of GKPID's capacity to bind the cortical marker protein - can be recapitulated by reintroducing a single historical substitution into the reconstructed ancestral GKPID. This change revealed and repurposed an ancient molecular surface that previously had a radically different function.-***-"A central issue in evolutionary biology is how complex systems originate through the action of mutation, drift, and natural selection. Tissue organization, spindle orientation, and the GKPID complex itself are all examples of complexity, defined as the integrated functioning of a system made up of differentiated, interacting parts. The GKPID complex can orient the mitotic spindle because of specific interactions between its component molecules and with other molecules in the cell and its local environment. In turn, the cellular phenomonenon mediated by this complex - regular orientation of the plane of cell division relative adjacent cells - allows the development and maintenance of organized, differentiated tissues, and this phenomenon in turn makes possible a higher level of biological complexity- the multicellular organism - from a collection of individual cells. Understanding the evolution of complexity at the molecular level can therefore help to illuminate the evolution of macroscopic complexity, including functions that are now crucial to animal biology per se. (my bold)-"Our work indicates that the GKPID complex was assembled stepwise through a process of molecular exploitation, in which old molecules with one function are recruited into a functional binding interaction with a newly evolved molecule. In this case, the GKPID, a duplicate of an ancient enzyme with an essential metabolic role in all life forms, already had the fortuitous capacity to bind the Pins protein, even before the latter protein appeared or subsequently acquired its relatively simple GKPID-binding linker motif (Figure 8). Once Pins did evolve this linker — along with its GoLoco motif, which interacts with G-protein complexes, which are also ancient (de Mendoza et al., 2014)— then a mechanism would have been assembled that could bring to specific locations on the cell cortex the GKPID and other proteins associated directly or indirectly with it, such as KHC-73 and astral microtubules, thus enabling externally-cued spindle orientation."-Comment: The bolded area is Michael Behe's definition of irreducible complexity. The mechanism involves a complex enzyme and compatible molecular surfaces which "fortuitously" fit together, also bolded. Fortuitous or planned?