Sequence conservation shows functional constraint indicates functional information. How many sequences will perform the specific function. If no new sequences are getting fixed in any mammal population then the amount of functional information is certainly high. Could Rum be right and some very different sequence work.
This is questionable as actin interacts with myosin. Maybe so in the unlikely event myosin mutates along with actin so it can escape its fixed position but unless the number of alternative functional sequences is enormous gpuccio’s hypothesis is correct. My original estimate was several hundred orders of magnitude less than the calculated number.