A sarcomere is the basic unit of a muscle and gives muscle its striated appearance. Sarcomeres contain overlapping actin and myosin motors that function in the contraction of each unit; multiply each sarcomere’s contraction many many times, and you’re able to lift your coffee mug. It was believed for a while that mature sarcomeres were very rigid structures, but recently that view has shifted. A recent paper describes the function of a sarcomere protein called Lmod in actin nucleation, suggesting its role in dynamic repair and remodeling of mature sarcomeres. Image above shows a chicken cardiac muscle cell with Lmod in green and Z-lines, which are the sarcomere's outside anchor points for actin, in red (α-actinin). Zoomed images of the boxed region show Lmod on either side of the center of the sarcomere, called the M-line, suggesting it is found at the ends of actin filaments.
Reference: Aneta Skwarek-Maruszewska, Malgorzata Boczkowska, Allison L. Zajac, Elena Kremneva, Tatyana Svitkina, Roberto Dominguez, and Pekka Lappalainen. Authors’ Molecular Biology of the Cell paper can be found here.