Final answer:
The lowest sequence identity that indicates regions of structural similarity between two proteins is typically around 25%. Protein structures can be conserved even with low sequence identity, as exemplified by certain studies, and therefore answer B) 25% is correct.
Step-by-step explanation:
In the context of comparing protein structures, the lowest sequence identity that usually indicates regions of structural similarity between two proteins is commonly around 25%. This low level of sequence identity can be sufficient because the three-dimensional (3D) structures of proteins are more conserved through evolution than the sequences of amino acids. 3D structures can remain similar even when the sequence identity is low, as similar folding can be maintained by different sets of amino acids if they share certain properties, such as size, hydrophobicity, or charge.
Specifically, the study mentioned regarding rabbit skeletal muscle actin and bovine ATPase shows that despite having only 39 locations in common, their 3D structures are nearly identical, suggesting that structural similarity can persist despite low sequence conservation. Protein motifs and domains, like PH domains, often remain recognizable and can conserve function and structure even with significant sequence divergence.
Therefore, the answer to the question is B) 25%, because structural similarity can still be significant even when sequence identity is relatively low.