α-Spectrin and integrins act together to regulate actomyosin and columnarization, and to maintain a monolayered follicular epithelium.

The spectrin cytoskeleton crosslinks actin to the membrane, and although it has been greatly studied in erythrocytes, much is unknown about its function in epithelia. We have studied the role of spectrins during epithelia morphogenesis using the Drosophila follicular epithelium (FE). As previously described, we show that α-Spectrin and ß-Spectrin are essential to maintain a monolayered FE, but, contrary to previous work, spectrins are not required to control proliferation. Furthermore, spectrin mutant cells show differentiation and polarity defects only in the ectopic layers of stratified epithelia, similar to integrin mutants. Our results identify α-Spectrin and integrins as novel regulators of apical constriction-independent cell elongation, as α-Spectrin and integrin mutant cells fail to columnarize. Finally, we show that increasing and reducing the activity of the Rho1-Myosin II pathway enhances and decreases multilayering of α-Spectrin cells, respectively. Similarly, higher Myosin II activity enhances the integrin multilayering phenotype. This work identifies a primary role for α-Spectrin in controlling cell shape, perhaps by modulating actomyosin. In summary, we suggest that a functional spectrin-integrin complex is essential to balance adequate forces, in order to maintain a monolayered epithelium.

The auto-inhibitory domain and ATP-independent microtubule-binding region of Kinesin heavy chain are major functional domains for transport in the Drosophila germline.

The major motor Kinesin-1 provides a key pathway for cell polarization through intracellular transport. Little is known about how Kinesin works in complex cellular surroundings. Several cargos associate with Kinesin via Kinesin light chain (KLC). However, KLC is not required for all Kinesin transport. A putative cargo-binding domain was identified in the C-terminal tail of fungal Kinesin heavy chain (KHC). The tail is conserved in animal KHCs and might therefore represent an alternative KLC-independent cargo-interacting region. By comprehensive functional analysis of the tail during Drosophila oogenesis we have gained an understanding of how KHC achieves specificity in its transport and how it is regulated. This is, to our knowledge, the first in vivo structural/functional analysis of the tail in animal Kinesins. We show that the tail is essential for all functions of KHC except Dynein transport, which is KLC dependent. These tail-dependent KHC activities can be functionally separated from one another by further characterizing domains within the tail. In particular, our data show the following. First, KHC is temporally regulated during oogenesis. Second, the IAK domain has an essential role distinct from its auto-inhibitory function. Third, lack of auto-inhibition in itself is not necessarily detrimental to KHC function. Finally, the ATP-independent microtubule-binding motif is required for cargo localization. These results stress that two unexpected highly conserved domains, namely the auto-inhibitory IAK and the auxiliary microtubule-binding motifs, are crucial for transport by Kinesin-1 and that, although not all cargos are conserved, their transport involves the most conserved domains of animal KHCs.

Polo-mediated phosphorylation of Maelstrom regulates oocyte determination during oogenesis in Drosophila.

In Drosophila, Maelstrom is a conserved component of the perinuclear nuage, a germline-unique structure that appears to serve as a site for Piwi-interacting RNA (piRNA) production to repress deleterious transposons. Maelstrom also functions in the nucleus as a transcriptional regulator to repress the expression of microRNA-7, a process that is essential for the proper differentiation of germline stem cells. In this paper, we report another function of Maelstrom in regulating oocyte determination independently of its transposon silencing and germline stem cell differentiation activities. In Drosophila, the conserved serine 138 residue in Maelstrom is required for its phosphorylation, an event that promotes oocyte determination. Phosphorylation of Maelstrom is required for the repression of the pachytene checkpoint protein Sir2, but not for transposon silencing or for germline stem cell differentiation. We identify Polo as a kinase that mediates the phosphorylation of Maelstrom. Our results suggest that the Polo-mediated phosphorylation of Maelstrom may be a mechanism that controls oocyte determination by inactivating the pachytene checkpoint via the repression of Sir2 in Drosophila ovaries.