Figure 3. Phosphorylated NMII light chains are organized as circular periodic structures persisting throughout the axon shaft.

(A) Immunolabeling of DIV8 hippocampal neurons with rabbit anti-pMLC (left) and rabbit anti-total MLC (right) using a secondary anti- rabbit Alexa Fluor 647 antibody. Scale bar: 5 μm. (B) Single colour SMLM of pMLC distribution in the AIS using a secondary Alexa Fluor 647 antibody. Scale bar: 500 nm. (C) Analysis of the periodicity related to (B). (D) 2D single colour SMLM of the axon shaft of a DIV8 hippocampal neuron immunostained for pMLC using a secondary Alexa Fluor 647 antibody. Scale bar: 500 nm. Periodic distribution in outer most regions of the axon shaft is highlighted with white ruler; inner distribution consistent with anchoring in different positions of adjacent actin rings is highlighted with red ruler. (E) Analysis of periodicity in outer most regions of the axon shaft (highlighted in white in D). (F) Analysis of inner periodic distribution (highlighted in red in D). (G) Two colour STED of a DIV8 hippocampal neuron immunostained against pMLC (red) and stained for actin (gray), using a STAR 580 secondary antibody and phalloidin 635, respectively. pMLC molecules, highlighted with arrowheads, co-localize with actin. Scale bar: 200 nm. The raw image was deconvolved using the CMLE algorithm (Huygens Professional, Scientific Volume Imaging). (H) Z projection of 3D two-colour SMLM of a DIV8 hippocampal neuron immunostained against βII-spectrin (red) and pMLC (green) using secondary antibodies labeled with Alexa Fluor 532 and 647, respectively. Molecules intercalating at outermost positions are highlighted using arrowheads (pMLC) and asterisks (βII-spectrin). Scale bar: 500 nm.