What is the Difference Between CD55 and CD59

The key difference between CD55 and CD59 is that CD55 is a protein that regulates the complement system by inhibiting aggregation of C3 convertases of the classical and alternative pathways and forming complement membrane attack complexes, while CD59 is a protein that regulates the complement system by preventing C9 from polymerizing and forming complement membrane attack complexes.

Complement membrane attack complex is a complex of protein formed on the surface of the pathogen cell membrane as a result of the action of the complement system of the host. The formation of the complement membrane attack complex triggers pathogen cell lysis. There are different molecules that can regulate the complement system and the formation of complement membrane attack complex. Therefore, CD55 and CD59 are two proteins that regulate the complement system by inhibiting the formation of the complement membrane attack complex.

CONTENTS

1. Overview and Key Difference
2. What is CD55 
3. What is CD59
4. Similarities – CD55 and CD59
5. CD55 vs CD59 in Tabular Form
6. Summary – CD55 vs CD59

What is CD55?

CD55, also known as complement decay-accelerating factor or DAF, is a protein that regulates the complement system by inhibiting aggregation of C3 convertases of the classical and alternative pathways and forming complement membrane attack complex. CD55 regulates the complement system on the cell surface. It usually recognizes C4b and C3b fragments formed during the activation of C4 (classical and alternative pathway) and C3 (alternative pathway). Interaction of CD55 with C4b of classical and lectin pathways interferes with the conversion of C2 to C2b, thereby preventing the formation of C4b2a (C3 convertase in classical and lectin pathway). On the other hand, when CD55 interacts with C3b of the alternative pathway, it interferes with the conversion of factor B to Bb by factor D. Thus, this prevents the formation of C3bBb (C3 convertase in the alternate pathway). Therefore, CD55 basically inactivates C3 convertases, which indirectly prevents the formation of the membrane attack complex.

Furthermore, this glycoprotein is encoded by the CD55 gene in humans. CD55 is also broadly distributed among hematopoietic and non-hematopoietic cells. In addition, it is also a determinant for the Cromer blood group system.

What is CD59?

CD59 is a protein that regulates the complement system by preventing C9 from polymerizing and forming a complement membrane attack complex. It is also known as MAC-inhibitory protein (MAC-IP), membrane inhibitor of reactive lysis (MIRL), or protectin. CD59 is a glycoprotein, and it is encoded by the CD59 gene in humans. It is a LU domain protein. Moreover, CD59 belongs to LY6/uPAR/alpha-neurotoxin protein family.

CD59 attaches to the host cells through a glycophosphatidylinositol (GPI) anchor. Normally, when complement activation leads to the deposition of C5b678 on the host cells, CD59 can prevent polymerizing and forming complement membrane attack complexes. CD59 can also signal the cell to perform active measures like endocytosis of the CD59-CD9 complex. Furthermore, the mutations affecting the GPI anchor reduce the expression of CD59, which results in a disease called paroxysmal nocturnal hemoglobinuria.

What are the Similarities Between CD55 and CD59?

• CD55 and CD59 are two proteins that regulate the complement system by inhibiting the formation of complement membrane attack complexes.
• Both are glycoproteins.
• Mutations affecting the GPI anchor reduce the expression of both CD55 and CD59, which result in a disease called paroxysmal nocturnal hemoglobinuria.
• They are extremely important for regulating host immune responses.

What is the Difference Between CD55 and CD59?

CD55 is a protein that regulates the complement system by inhibiting aggregation of C3 convertases of the classical and alternative pathways and forming complement membrane attack complexes, while CD59 is a protein that regulates the complement system by preventing C9 from polymerizing and forming complement membrane attack complex. Thus, this is the key difference between CD55 and CD59. Furthermore, CD55 is encoded by the CD55 gene in humans, while CD59 is encoded by the CD59 gene in humans.

The below infographic presents the differences between CD55 and CD59 in tabular form for side by side comparison.

Summary – CD55 vs CD59

CD55 and CD59 are two glycoproteins. CD55 regulates the complement system by inhibiting aggregation of C3 convertases of the classical and alternative pathways and forming complement membrane attack complex. On the other hand, CD59 regulates the complement system by preventing C9 from polymerizing and forming a complement membrane attack complex. So, his is the key difference between CD55 and CD59.

Reference:

1. “Decay Accelerating Factor.” An Overview | ScienceDirect Topics.
2. “CD59.” An Overview | ScienceDirect Topics.

Image Courtesy:

1. “Protein CD55 PDB 1h03” By Emw – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “CD59 2j8b” (CC BY-SA 4.0) via Commons Wikimedia