While nuclear actin was reported ~50 years ago, it’s
in vivo prevalence and structure remain largely unknown.
Here we use Drosophila oogenesis, i.e. follicle development, to
characterize nuclear actin. We find that three different reagents – DNase
I, anti-actin C4, and anti-actin AC15 – recognize distinct pools of
nuclear actin. DNase I labels monomeric or G-actin, and, during follicle
development, G-actin is present in the nucleus of every cell. Some G-actin is
recognized by the C4 antibody. In particular, C4 nuclear actin colocalizes with
DNase I to the nucleolus in anterior escort cells, follicle stem cells, some
mitotic follicle cells, and a subset of nurse cells during early oogenesis. C4
also labels polymeric nuclear actin in the nucleoplasm of the germline stem
cells, early cystoblasts, and oocytes. The AC15 antibody labels a completely
distinct pool of nuclear actin from that of DNase I and C4. Specifically, AC15
nuclear actin localizes to the chromatin in the nurse and follicle cells during
mid-to-late oogenesis. Within the oocyte, AC15 nuclear actin progresses from
localizing to puncta surrounding the DNA, to forming a filamentous cage around
the chromosomes. Together these findings reveal that nuclear actin is highly
prevalent in vivo, and multiple pools of nuclear actin exist
and can be recognized using different reagents. Additionally, our localization
studies suggest that nuclear actin may regulate stemness, nucleolar structure
and function, transcription, and nuclear structure. Such findings call for
further studies to explore the prevalence, diversity, and functions of nuclear
actin across tissues and organisms.