Spatiotemporal control of cortical centrin patterning by regionalized Sfi1 family scaffolding proteins in Stentor coeruleus.

Patterning is fundamental to the development and maintenance of organisms, ensuring functional and structural organization. While patterning is well studied at the level of multicellular organisms, single cells also form spatial patterns. Stentor coeruleus is a ciliate that has been a classical system for studying patterning and morphogenesis due to its distinctive shape and organization of easily visible cortical structures. The cortex of Stentor contains two cytoskeletal layers: microtubules and a network of centrin-family EF-hand proteins, which form a branched network in the anterior half of the cell and long, thick bundles known as myonemes in the posterior half. Sfi1 family proteins scaffold the assembly of centrin filaments throughout the eukaryotes and support contractile myonemal systems in some ciliates. In Stentor, a set of Sfi1 proteins upregulated during regeneration maintains anterior/posterior differences in centrin patterning, and this loss disrupts both regeneration and contraction. RNAi-mediated knockdown revealed that temporally expressed Sfi1 genes correlate with distinct stages of oral primordium growth. Additionally, the knockdown of Sfi1 isoforms that results in reduced myoneme cables impairs contraction, such that cells fail to contract in response to a stimulus or do not fully contract. These findings suggest a model in which regionalized differences in the patterning of cytoskeletal assemblies can be modulated by the regionalized localization of scaffolding proteins.