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Asymmetric placement of the photosensory eyespot organelle in Chlamydomonas is patterned by mother-daughter differences between the two basal bodies, which template the anterior flagella. Each basal body is associated with two bundled microtubule rootlets, one with two microtubules and one with four, forming a cruciate pattern. In wild-type cells, the single eyespot is positioned at the equator in close proximity to the plus end of the daughter rootlet comprising four microtubules, the D4. Here we identify mutations in two linked loci, MLT1 and MLT2, which cause multiple eyespots. Antiserum raised against MLT1 localized the protein along the D4 rootlet microtubules, from the basal bodies to the eyespot. MLT1 associates immediately with the new D4 as it extends during cell division, before microtubule acetylation. MLT1 is a low-complexity protein of over 300,000 Daltons. The expression or stability of MLT1 is dependent on MLT2, predicted to encode a second large, low-complexity protein. MLT1 was not restricted to the D4 rootlet in cells with the vfl2-220 mutation in the gene encoding the basal body-associated protein centrin. The cumulative data highlight the role of mother-daughter basal body differences in establishing asymmetry in associated rootlets, and suggest that eyespot components are directed to the correct location by MLT1 on the D4 microtubules., (© 2015 Wiley Periodicals, Inc.)

The photosensory eyespot of the green alga Chlamydomonas reinhardtii is a model system for the study of organelle biogenesis and placement. Eyespot assembly and positioning are governed by several genetic loci that have been identified in forward genetic screens for phototaxis-defective mutants. These include the previously described miniature-eyespot mutant min1, the multiple-eyespot mutant mlt1, the eyeless mutants eye2 and eye3, and two previously uncharacterized eyespot mutants, min2 and mlt2. In this study, effects of miniature- and multiple-eyespot mutations and their combinations on the localization and expression levels of the rhodopsin photoreceptor channelrhodopsin-1 (ChR1) and the localization of the eyespot-assembly proteins EYE2 and EYE3 were examined. min2 mutants assemble a properly organized, albeit nonfunctional, eyespot that is slightly smaller than wild-type; however, combination of the min2 and mlt1 mutations resulted in drastic reduction of photoreceptor levels. Both stationary-phase mlt1 and mlt2 cells have supernumerary, mislocalized eyespots that exhibit partial or total dissociation of the eyespot layers. In these mutant strains, photoreceptor patches in the plasma membrane were never associated with pigment granule arrays in the chloroplast stroma unless EYE2 was present in the intervening envelope. The data suggest that MIN2 is required for the photoreceptive ability of the eyespot and that MLT2 plays a major role in regulating eyespot number, placement, and integrity.

The eyespot of the unicellular green alga Chlamydomonas reinhardtii is a photoreceptive organelle required for phototaxis. Relative to the anterior flagella, the eyespot is asymmetrically positioned adjacent to the daughter four-membered rootlet (D4), a unique bundle of acetylated microtubules extending from the daughter basal body toward the posterior of the cell. Here, we detail the relationship between the rhodopsin eyespot photoreceptor Channelrhodopsin 1 (ChR1) and acetylated microtubules. In wild-type cells, ChR1 was observed in an equatorial patch adjacent to D4 near the end of the acetylated microtubules and along the D4 rootlet. In cells with cytoskeletal protein mutations, supernumerary ChR1 patches remained adjacent to acetylated microtubules. In mlt1 (multieyed) mutant cells, supernumerary photoreceptor patches were not restricted to the D4 rootlet, and more anterior eyespots correlated with shorter acetylated microtubule rootlets. The data suggest a model in which photoreceptor localization is dependent on microtubule-based trafficking selective for the D4 rootlet, which is perturbed in mlt1 mutant cells.

The eyespot of the biflagellate unicellular green alga Chlamydomonas reinhardtii is a complex organelle that facilitates directional responses of the cell to environmental light stimuli. The eyespot, which assembles de novo after every cell division and is associated with the daughter four-membered (D4) microtubule rootlet, comprises an elliptical patch of rhodopsin photoreceptors on the plasma membrane and stacks of carotenoid-rich pigment granule arrays in the chloroplast. Two loci, EYE2 and EYE3, define factors involved in the formation and organization of the eyespot pigment granule arrays. Whereas EYE3, a serine/threonine kinase of the ABC1 family, localizes to pigment granules, EYE2 localization corresponds to an area of the chloroplast envelope in the eyespot. EYE2 is positioned along, and adjacent to, the D4 rootlet in the absence of pigment granules. The eyespot pigment granule array is required for maintenance of the elliptical shape of both the overlying EYE2 and channelrhodopsin-1 photoreceptor patches. We propose a model of eyespot assembly wherein rootlet and photoreceptor direct EYE2 to an area of the chloroplast envelope, where it acts to facilitate assembly of pigment granule arrays, and EYE3 plays a role in the biogenesis of the pigment granules.

Assembly and asymmetric localization of the photosensory eyespot in the biflagellate, unicellular green alga Chlamydomonas reinhardtii requires coordinated organization of photoreceptors in the plasma membrane and pigment granule/thylakoid membrane layers in the chloroplast. min1 (mini-eyed) mutant cells contain abnormally small, disorganized eyespots in which the chloroplast envelope and plasma membrane are no longer apposed. The MIN1 gene, identified here by phenotypic rescue, encodes a protein with an N-terminal C2 domain and a C-terminal LysM domain separated by a transmembrane sequence. This novel domain architecture led to the hypothesis that MIN1 is in the plasma membrane or the chloroplast envelope, where membrane association of the C2 domain promotes proper eyespot organization. Mutation of conserved C2 domain loop residues disrupted association of the MIN1 C2 domain with the chloroplast envelope in moss cells but did not abolish eyespot assembly in Chlamydomonas. In min1 null cells, channelrhodopsin-1 (ChR1) photoreceptor levels were reduced, indicating a role for MIN1 in ChR1 expression and/or stability. However, ChR1 localization was only minimally disturbed during photoautotrophic growth of min1 cells, conditions under which the pigment granule layers are disorganized. The data are consistent with the hypothesis that neither MIN1 nor proper organization of the plastidic components of the eyespot is essential for localization of ChR1.

Chlamydomonas reinhardtii is a unicellular green alga whose lineage diverged from land plants over 1 billion years ago. It is a model system for studying chloroplast-based photosynthesis, as well as the structure, assembly, and function of eukaryotic flagella (cilia), which were inherited from the common ancestor of plants and animals, but lost in land plants. We sequenced the approximately 120-megabase nuclear genome of Chlamydomonas and performed comparative phylogenomic analyses, identifying genes encoding uncharacterized proteins that are likely associated with the function and biogenesis of chloroplasts or eukaryotic flagella. Analyses of the Chlamydomonas genome advance our understanding of the ancestral eukaryotic cell, reveal previously unknown genes associated with photosynthetic and flagellar functions, and establish links between ciliopathy and the composition and function of flagella.