<prompt> Which of the following is the commonest protein in mammalian cells?ActinTubulinKeratinNone of the above  <response>: Microfilaments are long solid fibers with a 4 to 6 nm diameter that are made up of actin. Although actin is most often associated with muscle contraction, it is present in all types of cells. It is the most abundant protein in mammalian cells, sometimes accounting for as much as 15 of the total protein in the cell. Ref: Ganongs Review of Medical Physiology 23rd edition, Chapter 2. 