LTP induction by structural rather than enzymatic functions of CaMKII.

Learning and memory are thought to require hippocampal long-term potentiation (LTP), and one of the few central dogmas of molecular neuroscience that has stood undisputed for more than three decades is that LTP induction requires enzymatic activity of the Ca2+/calmodulin-dependent protein kinase II (CaMKII)1-3. However, as we delineate here, the experimental evidence is surprisingly far from conclusive. All previous interventions inhibiting enzymatic CaMKII activity and LTP4-8 also interfere with structural CaMKII roles, in particular binding to the NMDA-type glutamate receptor subunit GluN2B9-14. Thus, we here characterized and utilized complementary sets of new opto-/pharmaco-genetic tools to distinguish between enzymatic and structural CaMKII functions. Several independent lines of evidence demonstrated LTP induction by a structural function of CaMKII rather than by its enzymatic activity. The sole contribution of kinase activity was autoregulation of this structural role via T286 autophosphorylation, which explains why this distinction has been elusive for decades. Directly initiating the structural function in a manner that circumvented this T286 role was sufficient to elicit robust LTP, even when enzymatic CaMKII activity was blocked.

Chromophore-Independent Roles of Opsin Apoproteins in Drosophila Mechanoreceptors.

Rhodopsins, the major light-detecting molecules of animal visual systems [1], consist of opsin apoproteins that covalently bind a retinal chromophore with a conserved lysine residue [1, 2]. In addition to capturing photons, this chromophore contributes to rhodopsin maturation [3, 4], trafficking [3, 4], and stabilization [5], and defects in chromophore synthesis and recycling can cause dysfunction of the retina and dystrophy [6-9]. Indications that opsin apoproteins alone might have biological roles have come from archaebacteria and platyhelminths, which present opsin-like proteins that lack the chromophore binding site and are deemed to function independently of light [10, 11]. Light-independent sensory roles have been documented for Drosophila opsins [12-15], yet also these unconventional opsin functions are thought to require chromophore binding [12, 13, 15]. Unconjugated opsin apoproteins act as phospholipid scramblases in mammalian photoreceptor disks [16], yet chromophore-independent roles of opsin apoproteins outside of eyes have, to the best of our knowledge, hitherto not been described. Drosophila chordotonal mechanoreceptors require opsins [13, 15], and we find that their function remains uncompromised by nutrient carotenoid depletion. Disrupting carotenoid uptake and cleavage also left the mechanoreceptors unaffected, and manipulating the chromophore attachment site of the fly's major visual opsin Rh1 impaired photoreceptor, but not mechanoreceptor, function. Notwithstanding this chromophore independence, some proteins that process and recycle the chromophore in the retina are also required in mechanoreceptors, including visual cycle components that recycle the chromophore upon its photoisomerization. Our results thus establish biological function for unconjugated opsin apoproteins outside of eyes and, in addition, document chromophore-independent roles for chromophore pathway components.