Lecture 14 - Melanosomes

Question 1

(i) What are Melanocytes?
(ii) What is the function of Melanosomes?

Answer 1

(i) Epidermal cells that produce and secrete melanin in response to stimuli (e.g., UV Exposure), which is packaged into melanosomes (membrane-bound organelles)
(ii) Melanosomes are transported along Melanocyte dendrites and taken up by keratinocytes, where they produce microparasol over nucleus

Question 2

What Transport Proteins is Melanosome Dispersion dependent on?

Answer 2

Myosin V, with mutants producing albinism due to perinuclear accumulation in melanosomes

Question 3

(i) What was the Co-operative Capture Model of Melanosome Transport?
(ii) How was this disproven?

(3 Points)

Answer 3

(i) MTs transported Melanosomes from the cell body to the dendrites, then Myosin V transported them to tips of dendrites via actin filaments
(ii) Disproven by:
* Inhibition of Myosin V function lead to significantly decreased melanosome dispersal, with MT presence driving greater clustering
* In Presence/Absence of MTs pigment dispersal is identical

Question 4

What proteins does Myosin V coordinate with to facilitate Melanosome Transport?

(3 Points)

Answer 4

1. Rab27a
2. Formin (FMN)
3. SPIRE

Question 5

Define SPIRE In terms of:
(i) Structure
(ii) Function

Answer 5

(i) Consists of:
* FYVE Domain - localises protein to membrane, also involved in dimerisation
* WH2 Domain - G-actin binding domains (bind to seed region of actin filament)
* KIND Domain - recruit Form

Question 6

What is Formin (FMN)? What is its Function?

(2 Points)

Answer 6

• Dimer which functions as processive capping protein, capping the barbed end (prevents capping proteins binding)
• Once Bound, feeds G-actin and profillin to growing end to promote polymerisation

Question 7

Summarise the Model of SPIRE/Formin/Myosin-V driven organelle transport

(3 Points)

Answer 7

• Rab27a interacts with SPIRE Box (SB) domain via its switch region, recruiting SPIRE to Melanosome
• SPIRE binds to seed region of Actin filament via WH2 domains, and recruits Formin, which facilitates actin polymerisation
• Myosin V recruited to melanosome moves along actin filament of another melanosome, leading to dispersal throughout cell