The p53 protein is an important molecule in cellular biology, widely recognized for its role as a tumor suppressor. It acts as a guardian, helping to maintain the integrity of the cell’s genetic material. This protein plays a significant part in regulating the cell cycle, ensuring proper cell division and growth.
The p53 Naming Discrepancy
When first identified, the p53 protein was observed to have an apparent molecular weight of 53 kilodaltons (kDa) when analyzed using a laboratory technique called SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis). This observation led to its designation as “p53,” with “p” standing for protein and “53” for its observed mass. However, this apparent weight is misleading; the actual molecular weight of human p53 is approximately 43.7 to 44 kDa.
The discrepancy arises because SDS-PAGE separates proteins primarily by size, but certain structural features can cause anomalous migration. In the case of p53, its unusually high content of proline amino acids contributes to this phenomenon. Proline residues introduce “kinks” or rigid bends in the protein chain that are not easily straightened out by the denaturing detergent SDS. This less linear, more compact, or rigid conformation causes the protein to move more slowly through the gel’s pores, making it appear heavier than it is.
Calculating the True Molecular Weight
Scientists determine the true molecular weight of p53 by bypassing the misleading results from gel electrophoresis. Its actual mass is calculated directly from its primary amino acid sequence. Human p53 is composed of 393 amino acids, each with a known molecular weight.
By summing the molecular weights of all 393 amino acids, along with considering any post-translational modifications like phosphorylation, the molecular weight can be derived. Modern analytical techniques, such as mass spectrometry, verify this calculated weight with high precision. This method provides a direct measurement of the protein’s mass, independent of its migration behavior in a gel.
Why p53’s Structure Matters
The sequence of 393 amino acids in p53 dictates its three-dimensional folded structure. This shape is important for p53 to perform its biological roles. The protein contains distinct domains, including an N-terminal transactivation domain, a central DNA-binding core domain, and a C-terminal oligomerization domain.
The DNA-binding domain allows p53 to recognize and attach to specific DNA sequences in the genome. This binding is a prerequisite for p53 to activate genes involved in halting cell division, initiating DNA repair, or triggering apoptosis (programmed cell death) in cells with irreparable damage. The oligomerization domain enables p53 to form a tetramer, which is also important for its activity.